I have been using my low side gauge to pull a vacuum, is this wrong?

I have been using my low side gauge to pull a vacuum, is this wrong?

YES. The low side gauge knows only atmospheric pressure and cannot sense moisture or non-condensables. A micron gauge is a heat sensing device that not only reads atmospheric pressure, but also measures the gases created by the vacuum pump as it boils the moisture. For example, if you were to pull a vacuum on an enclosed bottle of water, the low side gauge when pulling a vacuum will read a perfect vacuum. Using a micron gauge, it will immediately tell you with a high reading the you have a problem in you system.

My vacuum pump runs, but I cannot get a vacuum.

My vacuum pump runs, but I cannot get a vacuum.

The coupler between the shaft of the pump and the shaft of the motor is either broken or slipping. Make sure the set screws are tight on the flats of the two shafts.

Oil changing and dust removal – the essentials of vacuum pump care

While there are dry running vacuum pumps that do not require any oil, composites industry vacuum pumps are typically of the oil flooded rotary vane type. The reason the composites industry uses oil flooded pumps is because affordable oil-free vacuum pumps are usually limited to a maximum of about 85% vacuum. This is not enough for good quality resin infusion and pre-preg work, which usually requires pump vacuum levels of 95% and better – a level of vacuum easily achieved by an oil flooded rotary vane pump.

OK, so composites industry vacuum pumps have oil in them. Why is that a big deal?

  1. Vacuum pumps are always sucking contamination towards them. This contamination can be in the form of vapours such as organic resin solvents and water vapour extracted from the water absorbed into fabrics during manufacture and storage. (See Vacman’s Note “Vapour – the unseen enemy in composites”.)  Another form of contamination can be fine dust removed from the dry fabrics and pre-pregs, or particles of broken resin arising from the handling of vacuum lines containing cured resin. Solvent vapours passing through the pump tend to degrade the lubricating oil circulating around the pump. Water vapour entering into a vacuum pump, especially one that is cold, will condense into liquid water and this will coalesce with the pump oil. Water circulating with the pump oil will reduce the vacuum capability of the pump because water will boil readily on under vacuum on the inlet side of the pump. Pumps which are left idle with water contaminated oil will also corrode and wear more quickly when next restarted. As a further factor to be considered, there is a very fine filter inside the typical composites industry vacuum pump. This filter is called the exhaust oil mist filter or oil mist separator and it has to be fine enough to stop oil smoke being discharged from the pump. Contaminated oil will block the oil mist filter and raise the working pressure inside the pump, ultimately overloading the motor (and possibly rupturing the oil mist filter if it is very badly contaminated).
  2. Vacuum pumps for composites industry use are cooled by the air flow from the motor fan.  They will typically run with a pump oil temperature in the 90 ۰C to 110 ۰C (194 ۰F to 230 ۰F) range.  The pumps are designed to run at this temperature to minimise the affect of water vapour condensing in the pump oil.  (See point 1.)  While this temperature range will improve water vapour removal, higher temperatures will shorten oil and motor bearing life.  With clean external surfaces and in a moderate ambient temperature range of, say, 15 ۰C to 40 ۰C (59 ۰F to 104 ۰F), the oil temperature will sit within the desired range due to heat dissipation from the pump and motor surfaces.  However, if heat radiation from the pump and motor is reduced because of dust build-up on the pump and motor fins the pump and motor temperature will rise and both pump and motor life will be shortened.

Moral of the story – change the oil often and keep the pump clean externally

For optimum pump life, the 2 biggest favours you can do for your vacuum pump are to change the pump oil whenever it is contaminated and to keep the pump and motor surfaces clean.  Since vacuum pump oils are generally moderate cost and readily available compressor oils, oil changing should not be expensive, nor should it take much time.  As long as compressed air is available, cleaning the pump and motor will take very little time.

Oil change interval

While changing the oil at 500 hours is the normal recommendation, this interval should be the maximum time interval, not a hard and fast rule.  This is because oil contamination is highly variable, depending on individual process conditions.  In some cases, such as when a vacuum pump is used to bag down wood based fibre boards for mould making, it can be advisable to change the oil after each use because of the extremely high water vapour loading.  The most practical way to assess oil condition is to inspect the oil visible in the oil sight glass after the pump has been stopped for a few minutes.  If it is clear and “oily” in appearance (usually a pale gold colour) it will be fine.  If an opaque dirty black or brown, it will be loaded with dust or carbonised oil.  If the colour is opaque milky white/murky pale brown, it will be contaminated with water vapour.  If the latter and if the oil has been changed recently, you may be able to clean up the oil by leaving the pump run at full vacuum overnight.  If back to a clear gold colour in the morning, the oil will be fine.  If still murky, change it.

Note that using synthetic oil to extend the oil change interval is not recommended.  While synthetic oils can be beneficial in compressors and gearboxes, these applications do not suck in contamination to anywhere near the extent a composites industry vacuum pump does.  In our view, with vacuum pumps it is better to use a moderate cost all and change it frequently to avoid the build-up of contaminants.

Oil drain procedurevacmans-notes-a6-img1https://asiapumps.ir

  1. Make sure the pump is at operating temperature before changing the oil, ideally after running for at least 30 minutes, although 60 minutes is better.  The reason for getting the pump hot is to lower the oil viscosity as much as possible for maximum drainage.
  2. Switch off the pump and undo the oil drain plug.  If tight to undo, do not apply a long lever arm to the wrench.  Instead, give the wrench a sharp tap with a soft headed hammer.  The reason for a avoiding a long lever arm is because excessive torque may lift one end of the pump and break the rubber vibration isolators under the pump.  Bear in mind that the oil being drained will be hot – keep fingers clear of the hot oil!
  3. If the pump is on a mobile machine, such as a Vacmobile, lock the castors on the oil drain side of the machine and lift the other side of the machine for better oil drainage.
  4. While tilting the pump will improve drainage, some oil will remain in the pumping chamber.  If the oil is badly contaminated, the oil in the pumping chamber should also be removed.  To remove this remaining oil, replace the oil drain plug and tighten it hand tight.  Also make sure the oil fill plug is in place and hand tight. Briefly start and stop the pump – a second of running time will be long enough.  Once the pump has stopped, remove the drain plug again and drain the remainder of the oil.  Do NOT run the pump with either the oil drain plug or the filler cap removed – as that is likely to make a big mess!
  5. After draining the oil, refit and tighten the oil drain plug.

Oil filling procedure

  1. Check that the oil drain plug is in place and tight.
  2. Select the correct grade of oil for the particular pump model being serviced.
  3. Add oil slowly, with the pump in a level position.  For pumps fitted with a single round oil sight glass, the ideal level is between ½ way up the sight glass and the ¾ level.  With pumps with 2 sight glasses one above the other, a level about ½ way up the top one will be fine.  In either case, do not overfill.
  4. Refit the oil filler plug and tighten.
  5. Switch on the pump and check that the oil level remains below the top of the sight glass.

The reason it is important not to over fill the pump is because there is a possibility of liquid oil being blown up into the oil mist filter under high air flow conditions, e.g., when beginning to pump down.  If liquid oil is blown up into the oil mist filter, there is a risk that the filter will become saturated with liquid oil which the fine filter is not designed to handle.  If over-saturated with oil, the pressure inside the pump will increase excessively and there is a risk the element will rupture and need to be replaced.

Avoiding over-filling with oil is critical.  Always make sure that there is an air bubble of about ¼ of the oil sight glass diameter visible above the oil.

Cleaning dust from the external surfaces of the motor and pumpvacmans-notes-a6-img2https://asiapumps.ir

It will usually be most convenient if dust is removed from the motor and pump while the pump is running prior to an oil change.  Using a compressed air gun, blow dust from all accessible surfaces, but do not blow debris into the exhaust port of the pump.  One of the important areas to focus on is the fan end of the motor, as this is where dust and fibres tend to accumulate.  Dust will also tend to adhere to the blades of the motor fan, significantly reducing its efficiency.  Without poking the air nozzle into the moving fan blades, give the motor fan end a very thorough blow down.

Exhaust oil mist filter replacement

Replace the exhaust oil mist separator and clean or replace the gas ballast filter as soon as one of the following occurs:

  • New oil discolours quickly after changing.
  • Oil smoke is discharged from the exhaust port.  But don’t confuse oil smoke with water vapour being extracted from a wet material.  If any possibility of a high water vapour load, recheck for smoke after the pump has run at full vacuum overnight with the inlet valve closed.vacmans-notes-a6-img3-modified
  • Motor overload trips on starting or during the course of a job – but first check power supply voltage is correct and the power cord is in good condition.
  • After 1 year of daily use, or after 2 years of moderate use
  • After testing with a back-pressure gauge in accordance with the pump maker’s instructions. This is the surest method of testing, but it does require a pressure gauge and an understanding of the procedure.

Exhaust oil mist filter replacement is usually simple and will typically involve exposing the filter element as shown in the photo.

How to select lab vacuum pumps

How to select lab vacuum pumps

When selecting a vacuum pump for lyophilization, evaporation or concentration
applications, a vital consideration in pump performance is vapor tolerance. These
applications tend to involve high vapor flows that make extra demands of the
pumping capacity, so a pump that is designed to handle those vapors is important
to your success. But what do we mean by “handle those vapors?”
First things first. The first criterion in selecting a pump is to make sure that you have
one that produces vacuum in the most effective range for your application.
Vacuum Range
Most evaporative applications in the lab are best served by diaphragm pumps.
These can be made of chemical resistant materials and produce enough vacuum
to evaporate nearly every lab solvent (except DMSO) at room temperature. With the
addition of modest heat, even DMSO is manageable.
In contrast, lyophilization (freeze drying) requires vacuum that is deep enough to
induce sublimation – movement of a solvent directly from the solid state (e.g., ice) to
the vapor state. Effectively, since evaporative use of vacuum is directed at lowering
the boiling point, for sublimation we are trying to achieve a boiling point that is below
the freezing point (eutectic temperature). This takes much deeper vacuum than
diaphragm pumps can reach. For these applications, rotary vane pumps are the
most common choice

اثرات منفی فیلتراسیون خط مکش بر پمپ وکیوم

اثرات منفی فیلتراسیون خط مکش

برندان کیسی عملکرد فیلترها در سیستم هیدرولیک حفظ پاکیزگی مایعات است. با توجه به اینکه هدف از حفظ پاکیزگی مایعات ، بدست آوردن حداکثر عمر مفید از اجزای سیستم است ، ضروری است که درک کنیم برخی از مکانهای فیلتر می توانند نتیجه عکس داشته باشند ، خط مکش از جمله آنهاست. از نظر فیلتراسیون ، ورودی پمپ مکانی ایده آل برای فیلتر کردن محیط است. عدم وجود هم چنین سرعت سیال زیاد ، که ذرات به دام افتاده را مختل می کند و هم افت فشار زیاد روی عنصر ، که باعث کوچ ذرات از طریق محیط می شود ، باعث افزایش کارایی فیلتر می شود. با این حال ، این محدودیت ها ممکن است با محدودیت جریانی که این عنصر در خط ورودی ایجاد می کند و اثر منفی آن بر عمر پمپ ، غلبه کنند. فیلترهای ورودی یا مکش پمپ معمولاً به صورت صافی ۱۵۰ میکرونی (۱۰۰ مش) در می آیند که به داخل نفوذ ورودی پمپ در داخل مخزن پیچ می شود. محدودیت ناشی از صافی مکش ، که در دمای پایین سیال (گرانروی زیاد) و با مسدود شدن عنصر افزایش می یابد ، احتمال ایجاد خلا جزئی در ورودی پمپ را افزایش می دهد. خلا Ex زیاد در ورودی پمپ ممکن است باعث فرسایش کاویتاسیون و آسیب مکانیکی شود. فرسایش کاویتاسیون هنگامی که خلاial جزئی در خط مصرف پمپ ایجاد می شود ، کاهش فشار مطلق می تواند منجر به تشکیل حباب های گاز و / یا بخار درون مایع شود. هنگامی که این حباب ها در معرض فشارهای بالا در خروجی پمپ قرار می گیرند ، به شدت منفجر می شوند. فشارهای فروپاشی بیشتر از ۱۴۵۰۰۰ PSI ثبت شده است و در صورت بروز میکرودیزلینگ (احتراق مخلوط هوا / روغن) دمایی تا ۲٫۰۱۲ درجه فارنهایت ممکن است. وقتی حباب ها در نزدیکی سطح فلز فرو می ریزند ، فرسایش رخ می دهد (شکل ۱). شکل ۱٫ آسیب فرسایش کاویتاسیون به صفحه سوپاپ سخت شده مورد فرسایش حفره ای به سطح اجزای مهم آسیب می رساند و مایع هیدرولیک را با ذرات سایش آلوده می کند. کاویتاسیون مزمن می تواند باعث فرسایش قابل توجه شود و منجر به خرابی پمپ شود. آسیب مکانیکی هنگامی که خلاial جزئی در ورودی پمپ ایجاد می شود ، نیروهای مکانیکی ناشی از خلا itself خود می توانند باعث خرابی فاجعه بار شوند. ایجاد خلاuum در محفظه های پمپاژ یک پمپ محوری سوکت توپ پیستون و پد دمپایی را در کشش قرار می دهد. این اتصال برای مقاومت در برابر نیروی کششی بیش از حد طراحی نشده است و در نتیجه ، دمپایی از پیستون جدا می شود (شکل ۲). شکل ۲٫ دمپایی از پیستون خود جدا شده است نتیجه خلاuum بیش از حد در ورودی پمپ اگر نیروی کششی ناشی از خلا enough به اندازه کافی زیاد باشد ، یا در طی ساعتهای طولانی کار با اتصال مفصل توپی در زمان ورود ، کشش به صورت فوری اتفاق می افتد. صفحه نگهدارنده پیستون ، که وظیفه اصلی آن حفظ تماس دمپایی پیستون با صفحه swash است ، باید در مقابل نیروهایی که برای جدا کردن پیستون از دمپایی آن عمل می کنند ، مقاومت کند. این بار ناشی از خلا wear باعث تسریع در سایش بین دمپایی و صفحه نگهدارنده می شود و می تواند باعث پیچ خوردگی صفحه نگهدارنده شود. این اجازه می دهد تا دمپایی در هنگام ورودی تماس با صفحه swash را از دست بدهد ، و هنگامی که مایع تحت فشار در هنگام خروج روی انتهای پیستون عمل می کند ، آن را دوباره بر روی صفحه swash قرار می دهیم. این ضربه به دمپایی های پیستونی و صفحه سواش آسیب می رساند ، و به سرعت منجر به خرابی فاجعه بار می شود. در طرح های پمپ محور خمیده ، پیستون بهتر توانایی مقاومت در برابر نیروهای کششی ناشی از خلا را دارد. ساختار پیستون به طور کلی ناهموارتر است و توپ پیستون را معمولاً توسط یک صفحه نگهدارنده پیچ دار در سوکت شافت خود نگه می دارد. با این حال ، شکست کششی ساقه پیستون و / یا کمانش صفحه نگهدارنده هنوز هم می تواند در شرایط خلاuum زیاد رخ دهد. در طراحی پمپ پره ، پره ها باید از موقعیت جمع شده در روتور در هنگام ورودی امتداد داشته باشند. وقتی این اتفاق می افتد ، مایعات ورودی پمپ جای خالی روتور ایجاد شده توسط پره در حال گسترش را پر می کند. اگر خلا excessive بیش از حد در ورودی پمپ وجود داشته باشد – در پایه پره عمل می کند. این امر باعث می شود که پره ها در حین ورودی با حلقه بادامک تماس خود را از دست ندهند و پس از آنکه مایع تحت فشار در هنگام خروج بر روی پایه پره کار می کند ، آنها را دوباره بر روی حلقه بادامک قرار می دهند. این ضربه به نوک پره ها و حلقه بادامک آسیب می زند و به سرعت منجر به خرابی فاجعه بار می شود. پمپ های دنده از نظر مکانیکی کمترین حساس به نیروهای ناشی از خلاuum هستند. با وجود این واقعیت ، تحقیقات نشان داده است که گرفتگی صافی مکش ناشی از محصولات جانبی اکسیداسیون روغن صمغی می تواند عمر مفید پمپ دنده خارجی را حداقل ۵۰ درصد کاهش دهد. با توجه به احتمال صافی مکش برای آسیب رساندن به پمپ ، چرا اصلاً از آنها استفاده می شود؟ این س whenال کنجکاوتر می شود که در نظر بگیرید اگر مخزن و مایعات موجود در آن تمیز شود و کلیه هوا و مایعات ورودی به مخزن به اندازه کافی فیلتر شود ، مایعات موجود در مخزن حاوی ذرات سختی نیست که به اندازه کافی درشت شوند. صافی مشبک واضح است که بررسی استدلال های i

The Negative Effects of pump Suction Line Filtration

The Negative Effects of Suction Line Filtration

The function of filters in a hydraulic system is to maintain fluid cleanliness. Given that the objective of maintaining fluid cleanliness is to gain maximum service life from the system components, it is imperative to understand that some filter locations can have the opposite effect, the suction line is among them.

From a filtration perspective, the pump intake is an ideal location for filtering media. The absence of both high fluid velocity, which disturbs trapped particles, and high pressure-drop across the element, which forces migration of particles through the media, increases filter efficiency. However, these advantages may be outweighed by the flow restriction the element creates in the intake line and the negative effect this has on pump life.

Pump inlet or suction filters usually take the form of a 150-micron (100-mesh) strainer, which is screwed onto the pump intake penetration inside the reservoir. The restriction caused by a suction strainer, which increases at low fluid temperatures (high viscosity) and as the element clogs, increases the chances of a partial vacuum developing at the pump inlet. Excessive vacuum at the pump inlet may cause cavitation erosion and mechanical damage.

Cavitation Erosion

When a partial vacuum develops in the pump intake line, the decrease in absolute pressure can result in the formation of gas and/or vapor bubbles within the fluid. When these bubbles are exposed to elevated pressures at the pump outlet, they implode violently. Collapse pressures greater than 145,000 PSI have been recorded and if microdieseling occurs (combustion of air/oil mixture) temperatures as high as 2,012ºF are possible. When bubbles collapse in close proximity to a metal surface, erosion occurs (Figure 1).

https://asiapumps.ir
Figure 1. Cavitation Erosion Damage to
Case-hardened Valve Plate

Cavitation erosion damages critical component surfaces and contaminates the hydraulic fluid with wear particles. Chronic cavitation can cause significant erosion and lead to pump failure.

Mechanical Damage

When a partial vacuum develops at the pump inlet, the mechanical forces induced by the vacuum itself can cause catastrophic failure. The creation of a vacuum in the pumping chambers of an axial pump puts the piston-ball and slipper-pad socket in tension. This joint is not designed to withstand excessive tensile force and as a consequence, the slipper becomes detached from the piston (Figure 2).

https://asiapumps.ir
Figure 2. Slipper Separated from its Piston as
a Result of Excessive Vacuum at the Pump Inlet

This can occur either instantaneously, if the vacuum-induced tensile force is great enough, or over many hours of service as the ball joint is repetitively put in tension during inlet.

The piston retaining plate, the primary function of which is to keep the piston slippers in contact with the swash plate, must resist the forces that act to separate the piston from its slipper. This vacuum-induced load accelerates wear between the slipper and retaining plate and can cause the retaining plate to buckle. This allows the slipper to lose contact with the swash plate during inlet, and it is then hammered back onto the swash plate when pressurized fluid acts on the end of the piston during outlet. The impact damages the piston slippers and swash plate, leading rapidly to catastrophic failure.

In bent axis pump designs, the piston is better able to withstand vacuum-induced tensile forces. Piston construction is generally more rugged and the piston ball is usually held in its shaft socket by a bolted retaining plate. However, tensile failure of the piston stem and/or buckling of the retaining plate can still occur under high vacuum conditions.

In vane pump designs, the vanes must extend from their retracted position in the rotor during inlet. As this happens, fluid from the pump inlet fills the void in the rotor created by the extending vane. If excessive vacuum exists at the pump inlet – it will act at the base of the vane. This causes the vanes to lose contact with the cam ring during inlet, and they are then hammered back onto the cam ring as pressurized fluid acts on the base of the vane during outlet. The impact damages the vane tips and cam ring, leading rapidly to catastrophic failure.

Gear pumps are mechanically the least susceptible to vacuum-induced forces. Despite this fact, research has shown that suction strainer clogging caused by resinous, oil oxidation by-products can reduce the service life of an external gear pump by at least 50 percent.

Given the potential for suction strainers to damage the pump, why use them at all? This question becomes more curious when you consider that if the reservoir and the fluid it contains starts out clean and all air and fluid entering the reservoir is adequately filtered, the fluid in the reservoir will not contain hard particles large enough to be captured by a coarse mesh strainer. Clearly, examination of the arguments for installing suction strainers is required.

Trash Exclusion

The argument that suction strainers should be fitted to protect the pump from debris that enters the reservoir as a result of careless maintenance practices, is a popular one. Nuts, bolts, tools and similar debris pose minimal threat to the pump in a properly designed reservoir, where the pump intake is located a minimum of four inches off the bottom. When anecdotal evidence is presented that debris, which entered the tank through careless maintenance, did cause a pump failure, its weight is diminished on the basis that if a suction strainer had been fitted, the same neglect of its maintenance would have eventually resulted in the same outcome – premature pump failure. Notwithstanding the above, the preferred solution to this problem is to take action to prevent contaminants from entering the reservoir in the first place.

Warranty

Another popular misconception surrounding suction strainers is that their absence voids the pump manufacturers’ warranty. If a nut or bolt enters the pump through its intake causing it to fail, it is reasonable to expect that the manufacturer will deny warranty. It is also reasonable to expect the manufacturer to deny warranty if a pump failure is caused by particles smaller than the mesh of a strainer or by cavitation as a result of a clogged strainer. So if a pump fails through either contamination or cavitation, the manufacturer is unlikely to accept warranty – suction strainer or no suction strainer.

Where suction filters are fitted, the case for removing and discarding them is compelling. In most applications, the contamination control benefits these filters offer are strongly outweighed by the negative impact they can have on pump service life. In applications that demand their installation or where human barriers prevent their removal, precautions must be taken to prevent component damage.

If suction filtration is installed, a filter located outside the reservoir is preferable to a suction strainer. The inconvenience of servicing a filter located inside the reservoir is a common reason why suction strainers go unserviced – until the pump fails. If a suction strainer is used, opt for 60-mesh (240-microns) rather than the more common 100-mesh (150-microns). The strainer should be grossly oversized for the pump’s flow rate to ensure that pressure drop is minimized, even under the most adverse conditions. Regardless of the type of filter employed, it must incorporate a bypass valve to prevent the element from creating a pressure drop that exceeds the safe vacuum limit of the pump. A gauge or transducer should also be installed downstream of the filter to enable continuous monitoring of absolute pressure at the pump inlet.

ایجاد خلا (وکیوم) خیلی بالا

ایجاد خلا (وکیوم) خیلی بالا

تمیز توربو پمپ ها برای تولید خلا clean تمیز در محدوده ۱۰-۳ تا ۱۰-۱۰ hPa مناسب هستند. به لطف نسبت فشرده سازی بالا ، آنها با اطمینان روغن را از ناحیه ورودی پمپ های روغن دار و دور از گیرنده نگه می دارند. مدل هایی با محفظه های استیل ضد زنگ و فلنج CF قابل پخت هستند. این امر باعث می شود که این پمپ ها برای کاربردهای تحقیق و توسعه در مواردی که نیاز به خلأ بسیار زیاد است ، مناسب باشند. از توربوپمپ ها می توان برای تخلیه شناورهای بزرگ با پمپ های چرخشی پره ای به عنوان پمپ های پشتی استفاده کرد. در مورد پمپ های توربو ، پمپ های دیافراگم دو مرحله ای به عنوان پمپ های پشتی کافی هستند. اما به دلیل سرعت پایین پمپاژ ، زمان زیادی برای پمپاژ کشتی های بزرگتر طول می کشد. جریان گاز این ترکیب پمپ نیز توسط پمپ دیافراگم بسیار محدود می شود. با این حال این ترکیب یک راه حل بسیار مقرون به صرفه برای یک ایستگاه پمپاژ خشک است. این ماده اغلب برای طیف سنج های جرمی پمپ شده متفاوت و سایر کاربردهای تحلیلی یا تحقیق و توسعه استفاده می شود. اگر در منطقه پمپ پشتی به سرعت پمپاژ بالاتری نیاز است ، توصیه می کنیم از پمپ های ریشه ای چند مرحله ای از سری ACP یا برای فرآیندهای خلا chemical شیمیایی در صنعت نیمه هادی یا خورشیدی ، پمپ های پشتیبان با فرایند استفاده کنید. ایستگاه های پمپاژ متشکل از پمپ پشتی و توربوپمپ نیازی به شیر ندارند. هر دو پمپ همزمان روشن می شوند. به محض رسیدن پمپ پشتی به خلا fore پیشین لازم ، توربو پمپ به سرعت به سرعت اسمی خود می رسد و سریعاً ظرف را از فشار [Math Processing Error] <10-4 hPa با سرعت پمپاژ بالا تخلیه می کند. خرابی های مختصر برق را می توان با سرعت چرخش زیاد روتور از بین برد. در صورت قطع برق طولانی تر ، در صورت کاهش RPM ها به زیر حداقل سرعت ، می توان پمپ و گیرنده را به طور خودکار تخلیه کرد. تأثیراتی که در تخلیه شناورها نقش دارند در فصل ۲ شرح داده شده است. مسائل مربوط به ابعاد و همچنین محاسبه زمان تخلیه پمپ نیز در آن فصل شرح داده شده است. تخلیه محفظه های قفل بار تخلیه محفظه های قفل بار قطعاً هنگام انتقال قطعه های کاری که باید در یک فرآیند خلا treated تصفیه شوند ، به دست زدن تمیز نیاز دارد. اگر این موارد از فشار اتمسفر منتقل شوند ، ابتدا باید محفظه را از طریق یک خط بای پس تخلیه کنید. توربو پمپ در حال اجرا از طریق شیرآلات بین پمپ پشتی و محفظه متصل می شود. برنامه های تحلیلی امروزه در بسیاری از موارد از طیف سنج های جرمی در دستگاه های آنالیز استفاده می شود. مایعات اغلب در محفظه ورودی سیستم خلاuum تزریق و تبخیر می شوند. فشار در چند مرحله کاهش می یابد و اتاق های جداگانه توسط روزنه ها از یکدیگر جدا می شوند. از آنجا که هر محفظه باید پمپ شود ، هدف این است که جریان گاز از طریق شیرهای روی توربو پمپ از طریق ترکیبی ماهرانه از پمپ های پشتی و توربو پمپ ترکیب شود. توربو پمپ های اصلاح شده خاص با شیر برای برنامه های سری استفاده می شود. علاوه بر SplitFlow 50 که در فصل ۴٫۹٫۳ شرح داده شده ، راه حل های ویژه مشتری نیز می تواند ارائه شود. آشکارسازهای نشت هلیوم نیز مجهز به توربوپمپ هستند. در این حالت ، اغلب از اصل جریان متقابل استفاده می شود (به بخش ۷٫۲٫۱ مراجعه کنید). من. ه یک طیف سنج جرمی در سمت خلأ زیاد پمپ قرار دارد. با توجه به نسبت فشرده سازی کمتر توربوپمپ ها برای هلیم نسبت به نیتروژن یا اکسیژن ، پمپ به عنوان یک فیلتر انتخابی برای هلیوم عمل می کند. پمپ های دارای بار گاز زیاد در فرایندهای خلا vac توربوپمپ هنگام پمپاژ بارهای زیاد گاز برای فرآیندهای خلا دو مزیت دارد: در ابتدای هر مرحله فرآیند خلا clean تمیز ایجاد می کند و پس از آن می تواند گاز فرآیند را بدون هیچگونه برگشت مضر پمپاژ کند. در مرحله دوم ، هدف اصلی حفظ فشار خاصی است که در آن فرآیند خلا desired مورد نظر باید اجرا شود. در این فرایند ، توان تولید گاز و فشار کاری توسط برنامه مورد نظر تعیین می شود. من. ه یک مقدار جریان مشخص داده شده با یک جریان گاز معین پمپ می شود. علاوه بر این ، دستیابی سریع به خلا clean میانی تمیز هنگام تعویض قطعه های کار باید امکان پذیر باشد. از آنجا که این الزامات متناقض است ، باید یک توربو پمپ با اندازه کافی برای توان گاز مورد نیاز و خلا inter میانی مورد نیاز انتخاب شود. فشار فرآیند از طریق دریچه ورودی (مانند شیر پروانه ای) تنظیم می شود. نمونه ای از نحوه اندازه گیری این نوع ایستگاه پمپاژ در فصل ۲ نشان داده شده است. حداکثر بارهای مجاز گاز مشخص شده در داده های فنی باید به معنای بارهای مداوم مجاز باشد. این امر به شرط اطمینان از خنک سازی کافی مطابق با مشخصات و فشار پشتیبان متناسب با زیر حداکثر فشار پشتیبان بحرانی اعمال می شود. پمپاژ مواد خورنده و ساینده هنگام پمپاژ گازهای خورنده ، باید تدابیر لازم برای محافظت از موتور / مناطق تحمل و روتور به ویژه در برابر خوردگی گرفته شود. برای انجام این کار ، تمام سطوحی که با گاز خورنده تماس پیدا می کنند یا با روکش تهیه می شوند یا از fr ساخته می شوند

۱۳ عامل مشترکی که بر عمر پمپ وکیوم تأثیر می گذارد

۱۳ عامل مشترکی که بر عمر پمپ تأثیر می گذارد

بیش از ۴۵ سال پمپ به مدت طولانی طراحی شده است و کاربر پمپ را به گونه ای کار کرده و نگهداری می کند که منجر به نیم قرن کار بدون مشکل شود. در معادله کلی طول عمر قابل اعتماد پمپ ، تقریباً هر عاملی به کاربر نهایی – بخصوص نحوه عملکرد و نگهداری پمپ – بستگی دارد. به عنوان نمونه ، می توان انتظار داشت که یک پمپ استاندارد L-frame American National Standards Institute (ANSI) برای ۱۵ تا ۲۰ سال کار کند – و در بسیاری از موارد بیشتر از ۲۵ سال – اگر به درستی نگهداری شود و در نزدیکی بهترین عملکرد / طراحی کار کند نقطه. می توان انتظار داشت که یک پمپ پخش کننده چند مرحله ای با قدرت بالا در سرویس تغذیه دیگ بخار ۴۰ سال خدمات یا بیشتر ارائه دهد. برای طراحی خاص پمپ ، برخی از عواملی که کاربران نهایی می توانند برای طولانی شدن عمر پمپ کنترل کنند ، چیست؟ اگرچه این یک لیست جامع نیست ، اما ۱۳ فاکتور قابل توجه زیر موارد مهمی برای افزایش عمر پمپ هستند. ۱٫ نیروی شعاعی آمار صنعت نشان می دهد که بزرگترین دلیل خروج پمپ های سانتریفیوژ از کار ، خرابی یاتاقان ها و / یا مهر و موم های مکانیکی است. یاتاقان ها و مهر و موم ها “قناری های موجود در معدن ذغال سنگ” هستند – اینها شاخص های اولیه سلامت پمپ و منادی آنچه در سیستم پمپاژ اتفاق می افتد هستند. هرکسی که مدت زیادی در این صنعت بوده است احتمالاً می داند که بهترین روش شماره ۱ استفاده از پمپ در نزدیکترین نقطه کارایی (BEP) یا نزدیک آن است. در BEP ، پمپ با طراحی کمترین میزان نیروی شعاعی را تجربه خواهد کرد. بردارهای نیروی حاصل از تمام نیروهای شعاعی آغاز شده از کار دور از BEP در زاویه ۹۰ درجه روتور آشکار شده و سعی در انحراف و خمش شافت دارند. نیروی شعاعی بالا و انحراف شافت متعاقب آن کشنده مهر و موم های مکانیکی و عامل موثر در کاهش عمر تحمل است. اگر به اندازه کافی بالا باشد ، نیروی شعاعی می تواند باعث انحراف یا خم شدن شافت شود. اگر پمپ را متوقف کنید و میزان رانش در شافت را اندازه بگیرید ، به نظر نمی رسد مشکلی وجود داشته باشد زیرا این یک وضعیت پویا است ، نه یک حالت ایستا. شافت خم شده (انحراف) با سرعت ۳۶۰۰ دور در دقیقه (دور در دقیقه) دو بار در هر دور منحرف می شود ، بنابراین در واقع ۷۲۰۰ بار در دقیقه خم می شود. این انحراف با چرخه بالا باعث می شود سطح درزگیرها در تماس نباشند و لایه مایع مورد نیاز برای عملکرد صحیح آب بندی را حفظ کند. ۲٫ آلودگی روغن برای بلبرینگ ، بیش از ۸۵ درصد خرابی های بلبرینگ ناشی از نفوذ آلودگی است ، یا به عنوان خاک و مواد خارجی یا به عنوان آب. فقط ۲۵۰ قسمت در میلیون (ppm) آب ، عمر تحمل را با ضریب چهار کاهش می دهد. عمر مفید روغن حیاتی است. کارکرد پمپ می تواند شبیه کارکرد مداوم ماشین با سرعت ۶۰ مایل در ساعت باشد. با ۲۴ ساعت شبانه روز ، هفت روز در هفته ، طول نمی کشد که مایل ها را روی کیلومتر شمار بگذارید – ۱۴۴۰ مایل در روز ، ۱۰،۰۸۰ مایل در هفته ، ۵۲۴،۱۶۰ مایل در سال. برای کسب اطلاعات بیشتر در مورد مسائل روغن کاری ، به ستون های مربوط به روغن کاری در آوریل (در اینجا بخوانید) و ژوئن (اینجا بخوانید) ۲۰۱۵ Pumps & Systems 3. فشار مکش سایر عوامل کلیدی در طول عمر تحمل فشار مکش ، تراز بودن راننده و تا حدی کشیدگی لوله است. برای یک پمپ فرآیند افقی تک مرحله ای مانند مدل ANSI B 73.1 ، نیروی محوری حاصل از روتور به سمت مکش است ، بنابراین فشار مکش خنثی کننده – تا حدی و با محدودیت – در واقع نیروی محوری را کاهش می دهد ، که بارهای تحمل رانش را کاهش می دهد و به زندگی طولانی تر کمک می کند. به عنوان مثال ، یک پمپ استاندارد ANSI با فریم S با فشار مکش ۱۰ پوند بر اینچ مربع (psig) به طور معمول می تواند عمر تحمل شش تا هفت سال را داشته باشد ، اما با مکش ۲۰۰ psig ، عمر تحمل پیش بینی شده بهبود می یابد به بیش از ۵۰ سال ۴٫ تراز بندی درایور عدم انطباق پمپ و راننده ، بلبرینگ های شعاعی را بیش از حد بار می آورد. عمر تحمل شعاعی هنگامی که با مقدار عدم انطباق محاسبه شود ، یک فاکتور نمایی است. به عنوان مثال ، با یک عدم انطباق کوچک فقط ۰٫۰۶۰ اینچ ، کاربران نهایی می توانند در سه تا پنج ماه کارکرد ، انتظار نوعی مشکلات تحمل یا اتصال را داشته باشند. در ۰٫۰۰۱ اینچ عدم انطباق ، با این حال ، همان پمپ احتمالاً بیش از ۹۰ ماه کار خواهد کرد. ۵٫ لوله فشار کشیدگی لوله در اثر عدم هم ترازی لوله مکش و / یا تخلیه به فلنج پمپ ایجاد می شود. حتی در طراحی پمپ های قوی ، کشش لوله حاصل می تواند به راحتی این نیروهای بالقوه زیاد را به یاتاقان ها و محل قرارگیری مربوطه منتقل کند. نیرو (کرنش) باعث می شود که اتصالات یاتاقان دور نباشد و یا با دیگر یاتاقانها ناسازگار باشد به طوری که خطوط مرکزی در صفحات مختلف قرار گیرند. ۶٫ خواص مایعات خصوصیات مایع (شخصیت مایع) مانند pH ، ویسکوزیته و وزن مخصوص از عوامل اصلی هستند. اگر مایع اسیدی یا سوزاننده باشد ، قطعات خیس شده پمپ مانند پوشش و مواد پروانه باید در سرویس نگه داشته شوند. مقدار جامد

۱۳ Common Factors that Affect Pump Life

۱۳ Common Factors that Affect Pump Life

more than 45 years. The pump was designed to operate for a long time, and the user operated and maintained the pump in a manner that resulted in a half-century of trouble-free operation.

In the overall equation for reliable pump life expectancy, almost every factor is dependent on the end user—specifically, how the pump is operated and maintained. As an example, a standard L-frame American National Standards Institute (ANSI) pump can be expected to operate for 15 to 20 years—and in many cases longer than 25 years—if it is properly maintained and operated near the best/design operating point. A high-horsepower multistage diffusor pump in boiler feed service can be expected to deliver 40 years of service or more.

For a given pump design, what are some of the factors that end users can control to prolong a pump’s life?

While this is not an exhaustive list, the following 13 notable factors are important considerations for extending pump life.

۱٫ Radial Force

Industry statistics indicate that the biggest reason centrifugal pumps are pulled from service is the failure of bearings and/or mechanical seals. The bearings and seals are the “canaries in the coal mine”—they are the early indicators of pump health and the harbingers of what is happening inside the pumping system.

Anybody who has been around the industry very long probably knows that the No. 1 best practice is to operate the pump at or near its best efficiency point (BEP). At the BEP, the pump by design will experience the lowest amount of radial force. The resultant force vectors of all the radial forces initiated from operating away from the BEP manifest at 90-degree angles to the rotor and will attempt to deflect and bend the shaft.

High radial force and the consequential shaft deflection are a killer of mechanical seals and a contributing factor to bearing life reduction. If high enough, the radial force can cause the shaft to deflect, or bend. If you stop the pump and measure the runout on the shaft, nothing would appear to be wrong because it is a dynamic condition, not a static one.

A bent shaft (deflecting) operating at 3,600 revolutions per minute (rpm) will deflect twice per one revolution, so it is actually bending 7,200 times per minute. This high-cycle deflection makes it difficult for the seal surfaces to stay in contact and maintain the fluid layer required for proper seal operation.

۲٫ Oil Contamination

For ball bearings, more than 85 percent of bearing failures result from the ingress of contamination, either as dirt and foreign material or as water. Just 250 parts per million (ppm) of water will reduce bearing life by a factor of four.

Oil service life is critical. Operating a pump can be similar to operating a car continuously at 60 miles per hour. At 24 hours per day, seven days a week, it does not take long to put some miles on the odometer—۱,۴۴۰ miles per day, 10,080 miles per week, 524,160 miles per year.

For more information on lubrication issues, refer to my columns on lubrication in the April (read it here) and June (read it here) 2015 issues of Pumps & Systems.

۳٫ Suction Pressure

Other key factors for bearing life are suction pressure, driver alignment and, to some degree, pipe strain.

For a single-stage horizontal overhung process pump such as an ANSI B 73.1 model, the resultant axial force on the rotor is toward the suction, so a counteracting suction pressure—to some degree and with limits—will actually reduce the axial force, which decreases the thrust bearing loads, contributing to longer life. For example, a standard S-frame ANSI pump with a suction pressure of 10 pounds per square inch gauge (psig) can typically expect a bearing life of six to seven years, but at a suction of 200 psig, the expected bearing life will improve to more than 50 years.

۴٫ Driver Alignment

Misalignment of the pump and the driver overloads the radial bearings. Radial bearing life is an exponential factor when calculated with the amount of misalignment. For example, with a small misalignment of just 0.060 inches, end users can expect some sort of bearing or coupling issues at three to five months of operation; at 0.001 inches of misalignment, however, the same pump will likely operate for more than 90 months.

۵٫ Pipe Strain

Pipe strain is caused by misalignment of the suction and/or discharge pipe to the pump flanges. Even in robust pump designs, the resultant pipe strain can easily transmit these potentially high forces to the bearings and their respective housing fits. The force (strain) causes the bearing fit to be out of round and/or incongruent with the other bearings so that the centerlines are in different planes.

۶٫ Fluid Properties

Fluid properties (the fluid’s personality) such as pH, viscosity and specific gravity are key factors. If the fluid is acidic or caustic, the pump wetted parts such as the casing and impeller materials need to hold up in service. The amount of solids present in the fluid and their size, shape and abrasive qualities will all be factors.

۷٫ Service

The severity of the service is another major factor: How often will the pump be started during a given time?

I have witnessed pumps that are started and stopped every few seconds. Pumps in these services wear out at an exponentially higher rate than pumps that operate continuously under the same conditions. In these cases, the system design is in dire need of change.

Pumps with a flooded suction will operate more reliably than a pump in a suction lift scenario at the same conditions. The lift condition requires more work and offers more opportunities for air ingestion or worse—running dry. See my Pumps & Systems articles on submergence (April 2016, read it here) and self-primer problems (September 2015, read it here).

۸٫ NPSHA/R Margin

The higher the margin of net positive suction head available (NPSHA) is over net positive suction head required (NPSHR), the less likely the pump will cavitate. Cavitation will create damage to the pump impeller, and resultant vibrations will affect the seals and bearings.

۹٫ Pump Speed

The speed at which the pump operates is another key factor. For instance, a 3,550-rpm pump will wear out faster than a 1,750-rpm pump by a factor of 4-to-8.

۱۰٫ Impeller Balance

An unbalanced impeller on an overhung pump or on some vertical designs can cause a condition known as shaft whip, which deflects the shaft just as a radial force does when the pump operates away from the BEP. Radial deflection and whip can occur at the same time. I always recommend the impeller be balanced at least to International Organization for Standardization (ISO) 1940 grade 6.3 standards. If the impeller is trimmed for any reason, it must be rebalanced.

۱۱٫ Pipe Geometry

Another important consideration for extending pump life is the pipe geometry, or how the fluid is “loaded” into the pump.

For example, an elbow in the vertical plane at the pump’s suction side will induce fewer deleterious effects than one with a horizontal elbow. The impeller is hydraulically loaded more evenly, so the bearings are also loaded evenly.

Suction-side fluid velocity should be kept below 10 feet per second. I recommend keeping velocities below 8 feet per second, and 6 is even better (assuming non-slurry fluids). Laminar flow in lieu of turbulent will affect how the impeller is loaded and change the rotor dynamics.

۱۲٫ Pump Operating Temperature

Whether hot or cryogenic, the pump operating temperature—and especially the rate of temperature change—will have a large effect on pump life and reliability. The temperature at which a pump operates is important, and the pump needs to be designed to operate there. More important, however, is the rate of temperature change. I recommend (I am conservative) the rate of change to be managed at less than 2 F per minute. Different masses and materials expand and contract at different rates, which can affect clearances and stresses.

۱۳٫ Casing Penetrations

While not often considered, the reason casing penetrations are an option rather than a standard on ANSI pumps is the number of pump casing penetrations will have some effect on pump life because these sites are prime for the setup of corrosion and stress risers.

Many end users want the casing drilled and tapped for drains, vents, gauge ports or instrumentation. Every time you drill and tap a penetration in the casing, it sets up a stress riser in the material that becomes an origin source for stress cracks and presents a site for corrosion to initiate.

Succeed at Vacuum System Troubleshooting

Succeed at Vacuum System Troubleshooting

Understand the causes of common problems and how to address them.

By Keith Webb, Tuthill Vacuum & Blower Systems

When the desired vacuum condition isn’t provided at a process plant, production often comes to a halt and all eyes become focused on the vacuum pump as the root cause of the problem. However, the vacuum pump usually isn’t culprit. In almost all cases, either: 1) the pump is being operated in a condition for which it never was intended, 2) one or more of the user’s interface points with the pump (suction/discharge lines, water supply, process contaminant, etc.) are being operated outside of design parameters, or 3) the vacuum chamber or vacuum lines were improperly specified. Each vacuum pumping technology will react differently to various conditions, so it’s not possible to offer a “one size fits all” answer to the problem. The following is a guide to systematically identifying the root cause of the most common problems and correcting them based on general vacuum system recommendations as well as technology-specific issues.

Let’s start by noting that vacuum technologies found at plants generally fall into two categories: wet and dry. The terms “wet” and “dry” refer to whether or not the user’s process gas comes into contact with a liquid as the gas passes through the vacuum pump. Wet technologies utilize a liquid to create a seal between the discharge and the suction of the pump to minimize the “slip” of gas backwards from the discharge to the suction and increase volumetric pumping efficiency. Dry technologies have no liquid contact with the process gas. Table 1 lists common vacuum equipment of both types.

 

 

General Recommendations

The following points apply to all vacuum systems regardless of pump type:

Vacuum leaks. All vacuum systems have some amount of air-in leakage, which may or may not be known at the time the vacuum pump is sized. Excessive system leaks result in reduced process gas pumping capacity because the pump must move not only the process gas from the vacuum chamber but also the air-in leakage. Leaks occur at the joints of the vacuum lines and at the vacuum chamber. To avoid excessive air-in leakage, bear in mind the general recommendations of operating pressure ranges for various piping materials and joining methods detailed in Table 2. Note that actual limits will depend upon the skill level of assembly personnel.

Vacuum pump or system problem? You must determine if the issue is caused by the pump or by other equipment in the vacuum system. To find out, mount an isolation valve and an accurate vacuum gauge in-line as near to the suction connection of the vacuum pump as possible. Close the isolation valve and then measure the ultimate vacuum (also called blank-off) performance of the pump. Compare the measured vacuum to the manufacturer’s published ultimate vacuum value. A value reasonably close to the published one indicates the issue stems from leaks or outgassing in the vacuum system.

 

 

Excessive pump discharge or backpressure. A vacuum pump is designed to discharge to atmospheric pressure or just slightly above unless the manufacturer specifically designates it a compressor. As the discharge pressure of the pump increases above atmospheric pressure, this raises the differential pressure across the pump, resulting in:
• higher pump temperature and possible overheating, leading to pump seizure; and
• increased current draw and subsequent overheating of the electric motor or an overload/fuse/breaker fault.

Improperly sized suction and discharge lines. Sizing of system piping significantly affects pump performance and should be performed by qualified vacuum engineers. However, to avoid problems, apply the following guidelines:
• Suction and discharge lines never should be smaller than the suction or discharge connection size on the vacuum pump.
• For every 50 ft of suction or discharge piping, increase the pipe size by one nominal pipe diameter. Example: A vacuum pump has a 2-in. inlet connection. The suction line between the pump and the vacuum chamber is to be 70 ft long. To avoid restrictions to gas flow and pumping performance issues, increase the vacuum line to 3 in.

Isolation of pumps operated in parallel. Many vacuum pump installations consist of multiple pumps operating in parallel and utilizing a common suction and discharge header. For these type of installations, isolate idle pumps from those in operation at the suction and discharge. Failure to isolate the offline pumps may result in: 1) discharge gas from the operating pumps entering an idle pump and contaminating it, and 2) creation of vacuum in the idle pump and a resulting liquid back-stream into the vacuum lines and chamber.
Now, let’s look at specific issues that might affect particular equipment.

Liquid Ring Pumps

Several possible operating conditions can cause insufficient vacuum in liquid ring (LR) pumps. The most common are:
• too high sealant vapor pressure;
• incorrect sealant flow rate; and
• process contamination of the sealant (in full sealant recovery systems).

Too high sealant vapor pressure. A LR pump utilizes a sealant. Most commonly this is water but other liquids may be used based on the specific application of the pump. Generally, the lower the temperature of the sealant, the lower its vapor pressure, which results in increased pumping capacity and deep vacuum performance. In addition, as the process vacuum level approaches the sealant’s vapor pressure, the sealant will begin to flash from the liquid to the vapor phase (cavitation), subsequently displacing the pump’s capacity. Utilize sealant temperature/capacity correction factors from the specified LR pump manufacturer to properly size the pump.

As a rule of thumb, to avoid pump cavitation select a sealant whose vapor pressure, Pv, at operating temperature is less than half of the required vacuum level, P1, as measured at the pump inlet. For instance, the Pv of water at 60°F (15°C) is 13.3 mm Hg absolute. Therefore, the lowest vacuum operating pressure for the pump would be:

P1 = (۲)(۱۳٫۳) = ۲۶٫۶ mm Hg

Operating the vacuum pump’s suction pressure below this level will result in cavitation of the water within the pump that ultimately can damage the pump’s impeller (Figure 1).

 

 

Water at too high a temperature supplied to the pump directly as sealant or indirectly as coolant to the heat exchanger of a full sealant recovery system will increase the sealant’s vapor pressure. As the vapor pressure increases, this value may approach the vacuum level of the pump and cause the sealant to flash and reduce the pumping capacity. In many cases, the use of cooling tower water in high ambient temperature climates (>95°F or 35°C) results in significant capacity reduction. Figure 2 illustrates the capacity reduction when operating a pump at 75 torr should water sealant become much hotter than the desired 60°F.

Incorrect sealant flow rate. Each model of a particular manufacturer’s LR pump has a specific sealant flow rate requirement to achieve the published vacuum performance. Regulate the sealant flow to within approximately ±۵% of the published requirement. Simple and inexpensive flow control devices are available to regulate this flow.

If too much sealant is fed to the vacuum pump, the volume of the liquid ring within the pump will increase. This will reduce the volume of the rotor available for the pump to move process gas and the pump will lose pumping capacity, resulting in a loss of vacuum.

If too little sealant is fed to the vacuum pump, the liquid ring volume will decrease. The liquid ring no longer will be able to create the necessary seal between the rotor and the housing, allowing internal “slip” of the discharge gas back to suction and resulting in reduced pumping capacity and loss of vacuum.

 

 

Process contamination of the sealant (in full sealant recovery systems). Such contamination can involve carryover of condensate or particulates.

During the process of moving gases from the vacuum chamber through the LR pump, the process gas will contact the sealant and subsequently may collect in the sealant. If the substance collects in the sealant liquid and has a vapor pressure higher than that of the sealant, it will enter the LR pump and flash from the liquid to the vapor phase, reducing the pump’s capacity. As an example, when using oil as the LR sealant, if water vapor is a carryover product from the process gas, the vapor will condense to liquid in the discharge separator tank and effectively increase the pump sealant vapor pressure and decrease capacity.

Carryover of particulates or other solids may clog sealant piping, strainers, heat exchangers, valves, etc., and restrict sealant flow to the vacuum pump, resulting in reduced pumping capacity and possible overheating of the LR pump.

Oil-Sealed Rotary Pumps

Some of the most common field issues experienced by oil-sealed rotary piston pumps and rotary vane pumps are:

• belt squeal/high amp draw at startup;
• inability of pump to blank-off/milky oil;
• back-streaming of oil into suction lines or vacuum chamber; and
• excessive oil mist discharge.

Belt squeal/high amp draw at startup. Belt squeal of a pump at startup can stem from: 1) improper belt tensioning, 2) cold oil temperature due to low ambient temperature, or 3) improper shutdown procedure.

Typically, a loose belt causes belt squeal. Check for looseness by starting the pump and observing the deflection of the belt during rotation. Do not apply belt dressing to V-belts such as those used on Tuthill vacuum pumps. If the belt appears to have excessive deflection, refer to the manufacturer’s product manual for proper tensioning instructions.

The next likely cause of belt squeal/high amps is attempting to start the pump in low ambient temperature conditions, typically <60°F (15°C). In this case, you must install oil preheaters to increase the oil’s temperature and reduce its viscosity so the internal components don’t create high torque on the shaft. It often makes sense to use a temperature switch to ensure the pump will not start until the heaters have raised the oil temperature enough.

Lastly, oil-sealed rotary piston pumps are particularly prone to improper shutdown. A pump shut down under vacuum will leave an excessive amount of oil in the cylinder. Then, when an operator attempts to start the pump, the cold viscous oil will create high torque on the pump shaft, resulting in high amp draw. Oil-sealed pumps require that the inlet pressure of the pump be increased sufficiently (typically >100 torr for no less than 15 sec.) to allow more gas flow through the cylinder of the pump, resulting in displacement of the oil in the cylinder back into the main oil reservoir.

 

 

Inability of pump to blank-off/“milky”oil. Oil-sealed vacuum pumps commonly fail to meet the published blank-off performance due to: 1) substitution of the manufacturer’s vacuum pump oil with an improper oil, or 2) condensable process vapors collecting in the oil.

Vacuum pump operators for various reasons may not use the manufacturer’s recommended oil. This often can result in failure to produce the deep vacuum results as published. Vacuum pump oils are formulated to have a vapor pressure significantly lower than the pump’s ultimate vacuum capability. If a higher vapor pressure oil is substituted, the pump will begin to create vacuum and reach the vapor pressure of the oil in the cylinder. When this occurs, the oil will flash to the vapor phase, displace the pump’s capacity and result in higher blank-off values. The only remedy is to use an oil that has a vapor pressure equal to or less than that of the manufacturer’s vacuum pump oil. Matching the recommended oil’s viscosity also is necessary.

Many processes such as vacuum drying contain moisture that will condense when it reaches the pump’s oil reservoir at atmospheric pressure. The visual result is “milky” oil. Typically, the liquid has a vapor pressure significantly higher than the pump’s ultimate pressure. As the condensed liquid is recirculated with the oil into the cylinder (under vacuum), it begins to flash to a vapor phase. This again results in a higher-than-published blank-off value. The solution is either to: 1) run the pump’s gas ballast valve open (off process) for 15–۳۰ minutes, allowing the incoming air to strip the moisture from the oil, or 2) change the oil more frequently. Note that failure to perform one of these procedures will result in excessive wear of the internals due to increased friction and heat and, thus, reduced pump life.

Back-streaming of oil into suction lines or vacuum chamber. This commonly stems from failure to vent the pump’s inlet prior to shutdown. As already noted, oil-sealed pumps require that the inlet pressure of the pump be increased sufficiently (typically >100 torr for no less than 15 sec.) to allow more gas flow through the cylinder of the pump, resulting in displacement of the oil in the cylinder back into the main oil reservoir.

Excessive oil mist discharge. This phenomenon typically occurs because: 1) the pump has been operated continuously at an inlet pressure greater than the manufacturer’s recommendation, or 2) the pump’s oil mist element has failed.

Oil-sealed pumps commonly are used to operate continuously at inlet pressures <10 Torr or for short pump-down cycles that don’t allow oil to saturate the pump’s oil coalescing element. If a pump is operated above the manufacturer’s recommended maximum for prolonged periods, the relatively high gas density will carry the oil into the mist element at rates beyond its maximum filtering capability. The result is oil discharge from the exhaust of the pump. The best way to avoid this situation is appropriate sizing of the pump for the system design to avoid high operating inlet pressures for prolonged periods.

The other possibility is that the pump’s oil mist element fibers have separated due to continuous saturation and high pressure differential, resulting in the escape of oil mist from the pump’s exhaust. Replacing the element commonly will solve the problem.

Dry Screw Pumps

The two most common issues related to the improper application or operation of dry screw vacuum pumps are:

• overheating and pump seizure; and
• high motor amp draw.

Note that while dry screw vacuum pumps all have some common features, the symptoms of each pump will be manufacturer and model specific.

Overheating and pump seizure. Dry screw vacuum pumps are susceptible to several potential causes of overheating. The more common are:
reduced cooling water flow/high cooling water temperature; high inlet gas temperature; and improper staging with a vacuum booster.

The dry screw pump is more sensitive to cooling water flow and temperature than other technologies. A reduction in cooling water flow rate below the manufacturer’s minimum recommendation or supply cooling water temperatures in excess of the manufacturer’s recommendation can result in thermal growth and, ultimately, seizure of the pump.

Because dry screw pumps have no internal liquids to absorb heat, their internal temperatures can range from 250°F to 450°F depending upon the screw design. So, they are sensitive to inlet gas temperatures; each pump has a manufacturer’s maximum inlet gas temperature rating. Unfortunately, this value sometimes isn’t considered during the selection process and, as a result, the pump might encounter entering gas temperatures that exceed this value, resulting in excessively high internal gas temperatures that cause thermal growth and subsequent pump seizure.

The sizing process of a pump with a vacuum booster requires consideration of several parameters. One of the most important when pairing a vacuum booster upstream of a dry screw pump is staging ratio. This is defined as the ratio of the volumetric flow rate of the vacuum booster, V1, to the volumetric flow rate, V2: SR = V1/V2. Applying Boyle’s Law: V1/V2 = P2/P1.

Because V1 always is greater than V2, the pressure between the booster and the dry screw pump, P2, always will be greater than the inlet pressure, P1, to the system. The gas compression across the booster results in a temperature rise of the gas that will enter the dry screw pump. Therefore, carefully consider this ratio to avoid exceeding the inlet gas temperature rating of the dry screw pump.

High motor amp draw. Many types of rotating machinery experience high motor amp draw. Usually the cause isn’t an issue with the motor but rather with the piece of equipment it is driving. In the case of dry screw pumps, high amp draw typically results from: excessive discharge pressure (as noted in the general section); process buildup in the machine; or internal contact due to the cooling water and inlet gas temperature noted above.

Excessive discharge pressure as well as cooling water and inlet gas temperature already have been addressed, so, let’s focus on process buildup in the machine. Many vacuum processes contain chemicals that combine at high temperatures to form sticky or tacky materials that attach and then “bake onto” the screws (Figure 3). Their buildup ultimately creates a “zero clearance” condition inside the pump. This contact within the pump leads to additional torque on the pump shaft, resulting in increased amp draw.

Consult the pump’s manufacturer for a recommended solution. Generally this will involve either: 1) knocking out or filtering the process gases upstream, or 2) supplying a cleaning flush. Option 1 is preferable in extending pump life. However, filtration units can be costly and will require continual maintenance. In addition, as the filter elements clog, a resulting loss of vacuum in the process chamber will occur.

The cleaning flush option avoids the cost of the filtration system but may pose its own operational issues that could result in damage to the pump. Moreover, there’s no guarantee of success with the flushing process. Proper choice of flushing medium is most important and requires determining whether a solvent is needed to dissolve material or if a mechanical cleaning fluid such as water will suffice; the pump manufacturer should approve the selection. When injecting a direct liquid flush into a dry screw pump, take care not to flood the pump’s screw chamber as this can result in the pump attempting to compress liquid and subsequent mechanical failure requiring a major rebuild of the machine. Lastly, when injecting a flushing liquid into the pump’s process chamber, elevate the pump’s inlet pressure sufficiently above the vapor pressure of the liquid to avoid flashing. Such flashing to vapor will compromise cleaning as well as potentially create freezing problems within the machine due to the Joule-Thompson effect.

Achieve Long-Term Success

The process of creating a successful vacuum installation consists of several steps:

• Determine the parameters of the entire cycle of the vacuum operation from startup to shutdown.
• Select the appropriate vacuum technology and material of construction to match the process vacuum and flow requirement and gases to be handled.
• Properly size the vacuum pumping equipment, vacuum chamber and suction and discharge lines.
• Commission and leak check the vacuum system and validate on the process.

The vacuum pumping technologies addressed in this article are time-proven and will give years of reliable service when appropriately applied and operated. However, when troubleshooting is required, the pointers provided here should help you properly diagnose and address issues.