Why Pump Pressure Lower Than Expected

Maybe this has happened to you: You install a new pump or put a pump back online after a repair or rebuild. Then you turn it on and the pressure is not what you expect.

Category: Standards, Blogs, PSM Newsletter September 23, 2021

Maybe this has happened to you: You install a new pump or put a pump back online after a repair or rebuild. Then you turn it on and the pressure is not what you expect. 

It happens more often than you might think. That is why we developed this checklist of some of the most common causes of low pressure when starting up a new or rebuilt pump.

Intake or inlet piping. Improper inlet conditions are one of the most common reason pumps fail to produce their expect pressure. Inlet issues may also cause ongoing reliability problems. 

One culprit is air entrainment or lack of prime.  It is important to make sure that system is designed so that liquid floods the pump to prime it. If not, operators must prime the pump separately for startup and make provisions to maintain liquid in the pump for subsequent starts, otherwise the pump will not develop pressure.   

For systems that operate under a vacuum or at negative pressures, poorly tightened bolts or a cut gasket could let air into the system. This is often difficult to identify, since negative inlet pressure does not produce a noticeable outward leak.  

A third potential cause of entrained air is insufficient submergence, which can allow an air core vortex to form and pull air into the inlet piping or suction bell.  

Additionally, non-uniform flow due to improper intake or inlet piping design may degrade pump performance and cause uneven loading. Take, for example, a pipe elbow or another fitting that disturbs flow. Placed too close to the pump, it may force the liquid to one side of the impeller, creating an unbalanced load that keeps the pump from meeting its specifications, and will likely result in poor reliability. 

Insufficient suction pressure. Low pressure at the pump inlet can cause cavitation, which occurs when the liquid entering the impeller eye drops below its liquid vapor pressure and vaporizes to form bubbles. Vapor in the impeller saps pump performance. When those bubbles reach higher-pressure regions of the impeller, they rapidly implode, causing noise and erosion damage. 

While your system may have adequate pressure in the piping, it may not be sufficient to prevent vaporization as acceleration into the impeller eye causes pressure to drop. This is an example of insufficient net positive suction head available (NPSHA) compared to the net positive suction head required (NPSHR) of the pump. Refer to ANSI/HI 9.6.1 Rotodynamic Pumps – Guideline for NPSH Margin for recommendations. 

Wrong direction rotation. Improper rotation is more common than you would think in newly installed pumps. If the pump starts up and it sounds loud, reaches about half to two-thirds of the pressure you expect, and has a very low flow rate, you should probably check if the pump shaft is spinning the right direction. Fortunately, in a typical three-phase motor pump, this is an easy-to-fix wiring problem. Swapping the two electrical leads should get the pump rotating in the right direction.  It is recommended that the direction of rotation is verified for all new or reinstalled pump motors prior to operating the system.

Unexpected fluid properties. A change in liquid viscosity will change the performance of the pump, with increasing viscosity resulting in reduced performance. The performance of most pumps is tested using clean water, then corrected for the liquids they will actually handle based on known mathematical relationships. Sometimes, however, the actual conditions do not match the conditions for which the performance has been corrected, which can leave the system owner looking for the cause of lower than expected pressure. 

Also, changes in liquid density will alter the relationship of pressure to head. Take, for example, water. At ambient temperature, it has a specific gravity of 1.0 and 10 psi of pressure will produce 23.1 feet of head. If a fluid’s liquid density is lower than expected, say 80 percent of water (which would give it a specific gravity of 0.8), it would take only 8 psi to produce 23.1 feet of head. Therefore, if the system pressure is different than expected, verify that the liquid properties match the liquid the pump’s performance curve is based on or has been corrected to.

System issues or location of the pressure measurement. Sometimes, a system might have less resistance to flow than expected, allowing the pump to operate at a higher flow rate and lower pressure. Conversely, if you are measuring pressure in a section of pipe away from the pump, at a higher elevation, or after a flow obstruction, there will be a pressure drop; therefore, pressure will be lower than at the discharge of the pump. 

Improper pump assembly. Pump rebuilds can be complicated and should be done by professionals familiar with the equipment and its design. Sometime the pump assembly may have a problem. The most common possibilities include incorrectly set clearances and missing or improperly installed wear rings

These are some common pump startup problems that could result in lower than expected pressure, but by no means all of them (Refer to table 1 located below for some additional considerations).  Troubleshooting the problem relies on an understanding of the pump and system interaction, taking some basic measurements and comparing them to the pump performance curve.  

You can find a guide by referring to the trouble shooting section of ANSI/HI 14.4 Rotodynamic Pumps for Installation, Operation and Maintenance Manuals. You might also want to check out ANSI/HI standards for pump piping (ANSI/HI 9.6.6), intake design (ANSI/HI 9.8), and effects of liquid viscosity on pump performance (ANSI/HI 9.6.7). 

Table 1:

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