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Imagine if you will…
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Dispelling the Myth that Factory Supplied Pumps come “Plug and Play”.
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As we near the end of 2021, we “regift” some stocking stuffer pump tips
to equip you for a reliable New Year.
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This month, we review the reasoning for multiple pump alignments and why there may actually be more required than you think!
At the factory… the motor is positioned on the base plate and checked for proper alignment to the pump. As soon as the unit (base/motor/pump) is loaded on the truck the alignment integrity is lost. Later the unit is moved from the truck or warehouse/staging area to the foundation, consequently any semblance of an acceptable alignment tolerance is placed in jeopardy.
Industry best practices (experienced engineers, millwrights, and rotating equipment experts) will tell you that a pump requires a minimum of at least four (4) alignment checks prior to startup/commissioning. There may be an additional 4 or 5 checks to be conducted in the full commissioning process for a possible total of nine (9).
Align the Pump 9 Times
When I tell people that they need to conduct 9 alignment checks in the process of pump commissioning, I receive a wide range of interesting comments…please first let me explain my statement.
One… the first alignment
As mentioned above; prior to shipment the pump is positioned on the base and centered within its bolt tolerance. The motor is then placed on the base and a “rough alignment” is conducted to ensure that a precision alignment is possible and can be successfully conducted later in the field.
There can be exceptions to this example, and it depends on the type and size of driver. The initial factory alignment may only cover the side-to-side adjustments and not the aspects of the vertical or angular offsets, as those will be addressed in the field. No matter how precise the alignment conducted at the factory and how carefully the equipment is transported … the alignment will change in the process.
Second alignment check
When the pump, motor and base are received at the site and initially set on the foundation the second alignment should be conducted to correct for changes in transit. It is critical that the second alignment be conducted prior to grouting or connecting the piping to the pump. Omitting this step is a common and expensive mistake.
Three… the third alignment
You may have thought you had the alignment completed before, but now that the base is grouted you will find that that the alignment has again moved out of specification.
The process of grouting the base will oftentimes warp or otherwise move the base and offset the alignment. During the grout curing process there is a large amount of heat and some expansion force involved that will potentially change the alignment.
Four… the fourth alignment
Now that the base is properly grouted and the driver is aligned to the pump, it is time to connect the piping to the pump. It is always better to “pipe away” from the pump and not from the system to the pump to avoid unwanted moments and forces. Done correctly, connecting the piping to the pump should not alter the alignment in any way. However, from my experience this is a common reason for the alignment to change out of specification and introduce unwanted stresses into the pump.
Jim Elsey's Pumps and Systems Articles
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Recently, we investigated numerous issues with mechanical seals failing in self-priming pumps on suction lift applications. In each instance the pump/system was not installed or operated properly. Root cause analysis suggested a misunderstanding of basic physics. We thought it would be beneficial to review a few fundamentals for pumps on suction lift installations.
A suction lift simply means the maximum level of the liquid to be pumped is physically below the centerline of the pump impeller. Most centrifugal pumps can operate with a suction lift if they are primed first. Primed means the suction line, pump casing and impeller are full of liquid and all of the air or non-condensable gases are removed.
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A centrifugal pump cannot “suck” or ‘lift” the liquid into itself. Atmospheric pressure is the force pushing the liquid into the pump for open systems. From this information we can conclude; the maximum suction lift at sea level with a perfect pump, a perfect liquid and a frictionless leak free system can approach 34 feet (Atmospheric pressure at sea level is 14.7 psia X 2.31 ? 34).
NPSH Available
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Compressor vs. Pump
During the priming process the displaced air has to go somewhere. Even a great centrifugal pump is a really poor compressor due to the difference in density between air and water (? 800). If there is a check valve on the pump discharge, a parallel pump in operation and or a residual vertical liquid column, the pump will not prime. The air has to be vented somewhere, usually back to the suction source.
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Submergence
The critical submergence must also be calculated so the pump does not create vortices and pull air into the pump. Even a self-primer has limits for air entrainment.
Final Note
You can have sufficient NPSHA and not enough submergence …and you can also have adequate submergence and not enough NPSHA.
If you are ever in doubt regarding a pump application, please contact your Regional Sales Manager and/or our engineering group for assistance.
For more information see my related articles on the topic:
• Calculate NPSHa for a Suction Lift Condition
• 10 Common Self Priming Pump Issues
• Guidelines for Submergence & Air Entrainment
If time and money were no object, the pump OEM would be very happy to design a pump specifically for the customer’s unique operating point;
it happens, but not very often.
In essence, all single stage centrifugal pumps are designed for one operating point on the curve. This point is commonly referred to as the Best Efficiency Point (BEP). All other operating points are a degree of compromise with efficiency, cavitation, radial thrust, and recirculation issues. Ignoring these stressors will shorten the life of the bearings and mechanical seals, thereby making the pump less reliable and more costly to operate.
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Of course, most end users don’t have just one operating point and normally they want to operate in a wide area that is commonly referred to as a safe or allowable operating region (AOR). Therefore, most pump applications will require operation away from the area of BEP.
Reliability Management Methods
Assuming the pump selection was the best compromise (choice) for the application there are methods to mitigate the potential for the consequential negative effects. All the methods come standard with the added cost of reduced efficiency, but that cost can often be an acceptable tradeoff for reliability and reduced maintenance costs.
Some common mitigation methods are as follows:
There are also common methods to manage the symptoms versus the problem:
Another Possibility – Low Flow
In cases where the pump is operating on the left side of the curve it may be feasible to replace the pump with a low flow version. Standard single stage centrifugal pumps are typically of an expanding volute design. Expanding volute designs have the benefit of being more efficient, but the downside is increased radial thrust if you operate the pump away from the design point.
Low flow pumps utilize a circular casing where the casing is concentric to the impeller. By utilizing this design the radial thrust component can be appreciably reduced by amounts, normally in the range of 65 to 85%. In many cases using a low flow pump will reduce the need and cost for the more robust shaft (lower L over D ratio) and bearing system.
These are just some of the more common methods to reduce stress on the pump. If you are faced with an application that seems to have a “no right fit pump solution” Summit Pump can offer assistance.
We are your Best Value by
“providing quality pumping
products in a timely manner,
at a fair market price.”
Last month we reviewed basic vacuum principles.
The reason was to better understand how to correctly measure the differential pressure across an operating pump.
Understanding your suction pressure and knowing how to read a vacuum is critical in identifying performance issues. In the field, instruments measure in units of PSI …whether gauge, absolute or vacuum. When diagnosing issues, PSI must be converted to head in order to calculate Total Dynamic Head (TDH).
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Total Dynamic Head
Consider Total Dynamic Head as the amount of energy the pump converts from suction to discharge. This information is critical in identifying where the pump operates on the performance curve.
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To illustrate reading vacuum or pressure on the suction side of the pump, we will examine both a flooded suction and a suction lift application. Each example will assume the same pump, the same liquid and the same operation speed. The differential pressure of the pump will be the same for each example.
Note: For simplicity, we will not convert pressure readings to head in these examples, but understand the conversion mentioned above must be done in order to calculate TDH. For a more in-depth explanation, see my full article on the topic.
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Suction Lift
In Suction Lift, the pump must generate a low enough pressure at the eye of the impeller to have the fluid pushed into the pump by atmospheric pressure. Energy is used to make this happen and results in less energy available for pressure at the discharge.
Notice the overall pressure readings are lower but the differential pressure (75 PSI) remains about the same in both flooded and suction lift applications (using the assumptions we made earlier).
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Take-a-ways
We are your Best Value by
“providing quality pumping
products in a timely manner,
at a fair market price.”