The concept of vacuum mystifies some people when encountered on pump applications. Today, we hope to simplify the problem.

Pressure vs. Vacuum

Even in a vacuum there remains some pressure… it is simply a pressure at a magnitude below the surrounding atmospheric pressure. Vacuum does not necessarily mean the absence of all pressure; vacuum can be any pressure between 0 PSIA and 14.7 PSIA.

As a refresher…we offer the following points:

• Vacuum or vacuum pressure measurements are characterized as either absolute or relative (gauge).
• Absolute pressure is measured from zero, which means a 100% or a perfect vacuum.
• Relative pressure measurements are in reference to the atmospheric pressure. So if you see the word gauge or vacuum in the description it is being measured relative to atmosphere.
• Absolute pressure = gauge pressure + atmospheric pressure.
• Absolute pressure can be zero, but it can never be negative.
• Absolute pressure at sea level is usually given as 14.7 PSIA.
• Atmospheric pressure changes with elevation referenced to sea level and changes with barometric pressure (weather). If you are using gauge pressure to measure pressure or vacuum, you need to state the atmospheric pressure at the time of the measurement.

Units, Scales and Terms 

In the pump world we often encounter two descriptive terms, “absolute” and/or “vacuum” assigned to the measurement scale. Further, the measurements may either be in units of inches of Mercury (in-Hg) or pounds per square inch (PSI) in absolute pressure (PSIA) or gauge pressure (PSIG).

A perfect vacuum, if measured in absolute terms is zero (0 inches Hg) but is 29.92 in-Hg V (-29.92 in-Hg G) if the measured units are deemed relative (vacuum or gauge).

Due to the differences in these two measurement methods you need to ask the equipment owner if the parameter measurement scale is absolute or relative? The methods are 100% opposite of each other and if not properly understood can lead to some big mistakes.

Application

If we had a container at a theoretical perfect vacuum (that is, we have removed every molecule and its components from within the vessel) then we could state that vacuum condition as 0 (zero) pressure absolute or 0 PSIA.

However, if we were to measure our perfect vacuum in inches of Mercury Vacuum or inches Hg V the reading would be 29.92 inches Hg V not zero PSIA as in the first part of the example.

Need More?

Stay tuned for part two in next month’s 60 seconds with Summit Pump for more information. If at any point you are confused by a vacuum application in the field and not sure how to proceed, just give us a call.

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We get calls from the field concerning issues with pumps that are experiencing performance issues. Typically we perform a triage scenario; we first figure out if the pump is on a suction lift arrangement and make sure the pump is notattempting to defy the laws of physics. Once that question is satisfied we then walk through the Net Positive Suction Head Available (NPSHa) calculationswhere about 60% of the time we find that there is sufficient NPSHa. After the NPSHa exercise we ask about critical submergence (SC). A frequent reply to the submergence question is that no one knows what submergence really is or they never thought that it could be an issue if the NPSHa was sufficient.

Net Positive Suction Head (NPSHa) and Critical Submergence are two different things, and while they are connected by the liquid’s static height at suction, they are two independent factors requiring consideration for a successful suction system design.

Note: That you can have sufficient submergence and not have sufficient NPSHa.
Note: That you can have sufficient NPSHa and not have enough submergence.

What is Critical (Required) Submergence? 

Submergence is defined as the distance (D) measured vertically from the surface of the liquid to the centerline of the inlet suction pipe. A more important term is the required submergence, also known as minimum or critical submergence (SC). Required submergence is the vertical distance—from the fluid surface to the pump inlet—required to prevent fluid vortexing and fluid rotation (swirling and or pre-swirl).

What happens if I don’t have enough submergence? 

Without proper submergence the pump will create a vortex that will introduce air to the impeller suction. Just 2% air entrainment will have a negative effect on the pump hydraulic performance because the air bubbles become trapped at the impeller eye blocking the fluid flow. At 4% to 6% the performance will drop significantly and at 12% the pump may bind. See my related Guidelines for Submergence & Air Entrainment article for more details.

How much submergence do I need?

There are many sources of information regarding this subject with accompanying submergence charts that you can refer to in pump reference books, ANSI/HI 9.8 and on web sources. Normally the guides will suggest that for every one (1) foot of liquid velocity on the suction entrance you will require one-half (0.5) of a foot of submergence. Recently revised empirical data from the Hydraulic Institute and my professional suggestion is that if you want to avoid pulling air into the pump altogether, then you should elect the more conservative approach of one (1) foot of submergence per one (1) foot of liquid velocity.

In the harsh realities of the real world you may not always have the luxury of the conservative one foot of submergence allowance per one foot of liquid velocity approach. If that is the case please contact us for the means and methods to work around the issue.

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MYTH: Factory Supplied Pumps are “Plug and Play”.

Pumps shipped from the factory are NOT ready
to be started when and as received in the field.

As an annual ritual I am compelled to remind pump industry people that 99.35% (approx.) of industrial centrifugal pumps do not arrive ready to run and play – unfortunately this “Plug and Play” pump industry myth continues to persist.

Overview

1. There is NO OIL in the pump.
2. The impeller may or may NOT be set to the proper clearance.
3. The driver is NOT aligned to the pump.
4. The direction of rotation on the motor has NOT been determined.
5. The mechanical seal is NOT set.

If you already know these 5 things and fully understand the significance, then you can stop here. If you don’t know or would like a refresher please read on.

Oil
A pump shipped from the factory will NOT have oil in the bearing housing. Someone at the site must add oil prior to startup.

Oil is considered a hazardous substance in the commercial shipping world, consequently it is a violation of several federal laws to ship oil in the pump… Yes, there are means and methods to overcome this issue, but it requires special shipping, more money and paperwork. Additionally, OEM pump manufacturers are not in the business of stocking the multitude of different oils that a customer may request.

Impeller Clearance
A pump shipped from the factory may or may not have the proper axial clearance when it arrives on site. The factory adjusts the clearance at a nominal setting for the pump type and size based on ambient temperature water specifications.

The factory does not know the liquid’s temperature or other properties for the operating system. Note: it is also very possible the settings could have been adjusted after it left the factory. Confirming the clearance in the field is both easy and a professional best practice. Why take the chance? Also, prior to running the pump is the perfect time to take the initial total axial movement readings for the maintenance records.

Alignment
The driver will NOT be aligned precisely to a pump shipped from the factory. The factory utilizes laser manufactured templates for layout and performs a series of nominal checks to ascertain that the motor can be precisely aligned to the pump. Even if the factory did align the driver to the pump in accordance with the highest standards… as soon as the skid is picked up/transported the precision alignment will morph to unacceptable levels.

To learn more about about pump alignment, please check with your regional sales manager or refer to my articles on this subject:

Does Your Pump Have an Alignment Problem?
19 Tips and Common Alignment Mistakes

Driver Direction of Rotation
A pump shipped from the factory will NOT have the coupling spacer installed because you must first complete the driver rotational check with the coupling (spacer) removed. Additionally, with the coupling removed the process to set the impeller and mechanical seal is simplified.

The factory has a 50/50 chance of guessing the correct electrical phase rotation at your local site. If the rotation is wrong, the pump quickly converts to an expensive pile of useless scrap metal shortly after startup.

Mechanical Seal Setting
Factory installed mechanical seals will NOT be set. The pump comes with the seal clips in place (sleep position) to ostensibly preclude damage to the seal during shipping and handling. Plus prior to setting the seal the impeller clearance will need to be checked/set and the alignment completed.

Summary

• Read the instructions
• Add the oil
• Set the impeller clearance
• Complete the alignment and rotational checks … then set the seal
• Install the coupling spacer and the OHSA guard
Need some assurance when commissioning your pump? Give your RSM a call and/or perhaps review this article on the subject:

The Basics of Pump Startup

Finally
A warning tag is attached to each pump to communicate these 5 key steps to the end user/installer. Of course these steps have always been stated in the Instruction and Operations Manual (IOM). The IOM is included with every pump and if misplaced can also be downloaded from our website.

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Thanksgiving…At this time of year in America we purposefully pause and offer thanks for all we have. Suppose for a minute that in a fictional and make-believe universe you were a pump; what would you give thanks for? We share some ideas.

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How fast was I going officer?

Speed is a critical limit for any pump, but even more so for Positive Displacement (PD) pumps.  The maximum allowable speed of a PD pump is determined by several factors including the viscosity and temperature of the pumpage. Other important factors are the level of abrasives in the product, acceleration head, and the Net Positive Suction Head Required (NPSH_R).

Commercially available and cost effective electric induction motors nominally operate at speeds well above the optimum PD pump speeds, consequently some method must be used to reduce the drive output speed. Direct drive is just not all that common in most Internal Gear Pump (IGP) and Progressive Cavity (PC) applications for this reason.

The boundary for PD pump speed will typically be managed with either a gear reducer or a Variable Frequency Drive (VFD) and/or a combination of the two. For even more precise flow modifications a servo motor can be used in conjunction with both a gear reducer and a VFD.
Speed Kills

As the product temperature and/or abrasive concentrations increase, the pump should be operated at even slower speeds to reduce the inevitable wear and increase reliability. This may also mean a bigger and slower pump is required. Pump wear is exponentially proportional to speed. Even for relatively small increases in speed the wear rate can increase by a factor of eight.

Prior to purchase, the allowable speed range for the pump should be reviewed so that the correct choice of materials and speed control are made to achieve the lowest Total Cost of Ownership (TCO) and Mean Time between Failures and Repairs (MTBF/R).

I recently received a phone call and subsequent email stating “my self-priming pump is not priming”. Both the phone call and the email were detailed with pictures and appropriate information to start the troubleshooting process.

The pump was a model 2796 MTO 6 X 6 – 13 with a 10.625” diameter impeller turning at 1750 RPM. The design point was 1000 GPM at 85 feet of head and the liquid was pond water at ambient temperature. The 6-inch suction piping was approximately 60 feet long and extended down to the pond with a check valve located only 30 feet from the pump. We did not know the depth or submergence of the suction piping into the pond. The suction “lift” was 23 feet. The issues with this application are at the end of the article.

In pump school we review self-priming “do’s and don’ts” and have developed
a check list of items to review if you are having a problem:

And Now for the Rest of the Story –
The Issues with the Application

A wise man once told me you can not violate the rules of physics
and 95% of pump problems are on the suction side of the pump.

Issue #1 – NPSH_A is ? 6.5 feet. NPSH_R at BEP is 14.6 feet. NPSH_A must always be more than NPSH_R with as much margin as possible.

Issue #2 – Suction pipe is ? 60 feet. Too much air to evacuate.

Issue #3 – Submergence Unknown. Minimum submergence required is ? 8 feet (without a bell mouth setup).

Issue #4 – Check / Foot Valve located 30 feet from the pond. If you are going to use a foot valve it should be located at the bottom end of the suction pipe.

As a general guideline, if your pump takes more than four minutes to prime than you should shut the pump down and look for and correct the cause of the problem.

For More Information
Reference These Pump & Systems Articles
by Jim Elsey:

10 Common Self Priming Pump Issues
Guidelines for Submergence & Air Entrainment
Calculate NPSHa for a Suction Lift Condition

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One of the best ways to monitor pump reliability is to install two pressure gauges. One on the suction side and the other on the discharge side. With the pump curve in hand and knowledge of the speed and impeller size, the gauges will tell you exactly where and how well the pump is performing. If the pump is not on the curve refer to this article for more information.

All of us in the pump industry should be familiar with reading pump curves and we know that pumps have specific areas of operation. These areas have descriptions like; Shut Off, RIGHT or LEFT side of the curve, Run Out and Best Efficiency Point (BEP). If the pump is “running left” on the curve, this simply means that the pump is delivering relatively higher pressure and lower flow. “Running to the right” means a higher flow rate, but a decrease in discharge pressure. BEP is the point of optimum flow and efficiency. The bottom line is that without gauges you will not know where the pump is operating. Oftentimes a gauge reading is a more accurate performance indicator than a flowmeter.

Here are a few scenarios on why using gauges is important in maintaining and troubleshooting a pump.

1. Readings from both the discharge and suction gauges are a useful tool because the difference in pressures is proportional to the total head.
2. The pump will operate where the system curve intersects it. Your estimate of where that point truly is may be off.
3. A shutoff head test will provide useful information in relation to the pump’s health.
4. Static readings are useful in detecting a leak in the suction or discharge.

One last note: I frequently see gauges installed directly on the pump flanges; while a common practice this is not always the best location. For gauges to provide an accurate reading the gauge taps should be installed close to the pump, but preferably at a minimum of 3 to 6 pipe diameters away from the flanges.

If you don’t measure it, you can’t manage it.

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Today we will discuss ANSI pumps with a focus on Summit’s model 2196 product line.  

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Ever wonder why some impellers, fresh and new from the factory have holes in them? Or… why some impellers have those funny mini vanes on the back side? The short answer is to reduce the axial thrust in the pump and also to reduce the pressure in the stuffing box. 

Whenever you operate a centrifugal pump of any design there are always dynamic forces at play. A good engineer will address all of the forces in the pump and create designs to reduce and or eliminate the effects. There are several dynamics to deal with in the pump, but the two main ones are the axial and radial forces.

Axial forces are those that act on the rotor in a direction parallel to the shaft. These forces exist due to higher pressures acting on one side of a surface and/or acting on a larger area. The main surface area is the impeller shroud(s).

In large multistage pumps the axial force is mostly neutralized by the use of a balance drum. In other multistage pumps it can be managed by using the opposed impellers method. For example, in a 6 stage pump, 3 impellers face one direction and the other 3 face the opposite way.  Another example is the horizontal split case pump where the impeller has two inlet eyes that are opposed at 180 degrees to each other. The net effect is an almost balanced (negating) axial force.

ANSI style (B73.1) pumps are end suction types that use semi open impellers 99% of the time. These impellers have a shroud on one side only. This geometry makes them easy and less expensive to manufacture. However, a downside consequence is an impeller with higher unbalanced axial forces.

Under normal operating conditions there will be a much higher force on the back of the impeller than on the front. The resultant force will attempt to push the pump impeller towards the suction. The thrust bearing counteracts that force. It is not uncommon for a medium frame size ANSI pump to develop up to 850 pounds of force exerting in the direction towards suction. The axial force will lessen with higher suction pressures.

A designer could simply install bigger thrust bearings and not worry about the axial force, but the bigger bearings require bigger shafts and that requires bigger housings and all those things result in a pump that has both a bigger initial cost and a bigger maintenance cost.

The method most pump engineers use is to reduce the axial force behind the impeller. This is where impeller balance holes and or pump out vanes come into play as an axial force reduction method. You can’t get something for nothing so there is a small tradeoff with efficiency and power when using this approach.

For those of you that are interested in the details as to how and why pump out vanes and balance holes actually work, I will compose a more detailed article for Pumps and Systems magazine in the near future.

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We have all heard the term “flush” used when discussing mechanical seals. The definition of a flush is “a stream brought in from an external source to the mechanical seal.” This plan (API Plan 32) is almost always used in conjunction with a close clearance throat bushing. The flush fluid must be brought into the stuffing box at a minimum of 15 PSIG higher than stuffing box pressure.

The advantage is that the external flush fluid, when selected properly, can result in extended seal life. When an outside flush source is used, concerns regarding product dilution and/or economics must be considered by the user. The picture below shows API Plan 32 arrangement.

API Plan 32
Piping illustration used are copyright material and have been used with permission from AESSEAL

The confusion arises when our customers call a discharge recirculation (API Plan 11) a “flush”. Although this terminology is often used, this is where the confusion lies. This plan takes fluid from the pump discharge (or from an intermediate stage), through an orifice(s) and directs it to the seal chamber to provide cooling and lubrication to the seal faces. The advantage is no product contamination and piping is simple.

You must remember the fluid is coming back to the seal at a higher pressure. If the fluid contains particulate, you are bringing dirt and contaminants to the seal at high pressure. Think sandblaster. This is a good piping plan to control vapor pressure. Think hot water and flashing between the seal faces. The picture below shows a Plan 11 arrangement.

API Plan 32
Piping illustration used are copyright material and have been used with permission from AESSEAL

A primary factor in achieving highly reliable, effective sealing performance is to create the best fluid environment around the seal. Selection of the right piping plan and associated fluid control equipment requires a knowledge and understanding of the seal design and arrangement. As well as awareness of the fluids in which the seals operate and the rotating equipment to which they are fitted.

Provision of clean, cool face lubrication, effective heat removal and consideration of personnel and environmental safety, leakage management and controlling system costs are among the specific factors that must be considered. It is proven to prevent premature mechanical seal failure you must use a reliable seal support system.

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Snoring pumps… ever heard of such a thing?

I knew that pumps could run, burp, leak, stall and die (people “kill” them all the time),
but I didn’t know that pumps could snore.

Kidding aside, by definition a pump that is pumping a mixture of liquid and air is technically snoring. The term originates from the process noises associated with the phenomena.

Does your pump have the ability to pump normally, then operate dry for some length of time and then self-re-prime and return to pumping? And then…does it have the ability to repeat that process over and over? Probably not. Pump snoring is a condition that leads to reduced reliability and shorter pump life.

Most all centrifugal pumps DO NOT have the ability to pump liquids with air entrainment above 10% and almost never much above 14%. As a matter of fact, most all centrifugal pumps will have issues starting as low as 2% air entrainment. Note: that self-primers, recessed impeller (vortex) pumps and disc friction pumps can possibly pump mixtures at higher percentages.

Snoring is usually a term reserved for submersible pumps on dewatering applications at construction sites, but the phenomena can apply to most any centrifugal pump type and application.

Another application where this phenomena shows up is pumping a tank down to empty and/or for transfer. The snoring issue occurs frequently with batch process operations and if the operator (or the process control system) are “out to lunch” the pump consequently suffers mortal damage to the clearances, mechanical seals and bearings.

Sometimes it is not the operator, but the system design that creates the issue such as dissolved – air flotation (DAF) systems and waste water treatments that require additives such as surfactants, alcohols and soaps.

One of the main culprits for pump snoring is simply poor sump design, where the influent is dumped into the sump at elevations high above the liquid level at or near the pump suction intake without the benefit of weirs or baffles. This improper geometrical arrangement contributes to a “waterfall effect” pulling air into the liquid…it probably should be called the “water torture” method.

Other common causes for air entrainment are sumps that are too shallow, frequently experienced on cooling tower applications and in underground mining applications (minimum overhead space) where the air gets mixed into the liquid due to inadequate submergence thereby creating a vortex action.

Why do we care?
Entrained air is directly related to:

• Reduced pump performance, both head and flow and often to the point of stall
• Increase in vibrations and noise (the reduction in efficiency manifests as these)
• Overheating
• Higher incidents of shaft breakage

Furthermore…
Don’t confuse air entrainment with cavitation as they are two different things, but they can sometimes be related by a root cause and both can occur at the same time. Lastly, do not confuse dissolved air with entrained air.

Stop your pumps from snoring, don’t let them drink air before they go to bed.

For added background refer to my articles in Pumps and Systems magazine.
The links are below for your convenience.

How to Reduce or Eliminate Air Entrainment:
https://www.pumpsandsystems.com/how-reduce-or-eliminate-air-entrainment

Guidelines for Submergence & Air Entrainment
https://www.pumpsandsystems.com/guidelines-submergence-air-entrainment

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We are your Best Value by
“providing quality pumping
products in a timely manner,
at a fair market price.”