Issues with a Reduced Margin
The friction component of the NPSHa formula becomes increasingly large when the strainer becomes clogged. This is a negative component in the formula and as a result the reduction in available NPSH will cause the pump to cavitate.
If you look deeper into the cavitation phenomena, the reduction in flow caused by the restricted strainer will result in a higher velocity across the device. The fluid in the suction line must obey the law of conservation of energy, which states energy can neither be created nor destroyed, but it can be altered in form. This is best summarized in Bernoulli’s equation.
Bernoulli’s Law, simplified for this article, explains why the pressure will drop correspondingly as the velocity increases. The clogged strainer is why the velocity has increased in the first place. As the pressure drops, the vapor pressure will also be affected, and it is possible at some point for the liquid to change state and form a vapor. Centrifugal pumps are not capable of pumping air, vapor or non-condensable gases. If there is as little as 4 percent entrainment in the liquid, it can bind (vapor lock) the pump.
The higher the temperature of the liquid, the higher the possibility for this phenomena to occur.
I have covered details of these phenomena in four other articles in Pumps & Systems magazine.
- How to Reduce or Eliminate Air Entrainment (December 2017)
- Most Common Reasons for Air Entrainment in Pump Systems (December 2017)
- Guidelines for Submergence & Air Entrainment (April 2016)
- 10 Common Self Priming Pump Issues (September 2015)
Oil & Gas Applications
In oil and gas applications where the pump is upstream at the well head, there is simply no way to avoid the introduction of solid containments and dual phase fluids to the pump suction. It is extremely rare that a fluid coming out of the ground will be 100 percent gas free. A centrifugal pump cannot pump (compress) air or gas.
The comparative difference in the range of fluid densities is a factor of approximately 800. Different designs and sizes of pumps handle this issue in better ways than others, but at some level all centrifugal pumps will vapor lock and fail. The impeller eye will become blocked by the air, vapor or gas.
If the pump is a self-primer or simply a centrifugal pump placed in a lift condition application (externally primed), there can or will be performance issues should any restriction be placed in the suction line. Note that “lift” signifies that the level of the source liquid to be pumped is below the centerline of the pump impeller. The available NPSH will already be low because the static head component in the formula is now a negative quantity due to the lift condition. Further restrictions will add to the negative component of friction and for any temperature above ambient the vapor pressure component will also work against the pump.
“OK, I understand you, but I need to have strainers on the suction side.”
Assume you must have strainers on the suction side of the pump. Now what?
If there are strainers on the suction side of the pump, the best step you can take is to add instrumentation and continuously monitor the differential pressure (DP) across the strainer. The strainer will have a resistance coefficient assigned by the manufacturer. This is a great place to use an automated alarm system. The DP across the strainer for both clean and dirty conditions should be known. There must be a low value of DP for the new and clean strainer; I prefer less than 2 pounds per square inch gauge (psig). Also, you can compare the low clean value of DP to a higher value for a clogged strainer as indication for action.
Note: You have to know the head loss across the strainer anyway to do the NPSHa calculation. How else would you know if there is sufficient margin in your design?
If the DP across the strainer is not automatically monitored and alarmed, then an operator must check on a scheduled basis. All changes must be recorded and action must be taken if the DP is out of specification. Even a difference of 1 or 2 psig DP can be the difference between success and failure.
I suggest using a duplex gauge rather than two separate gauges, due to differences in system losses and gauge inaccuracies. A differential pressure transducer is normally better than a duplex gauge.
Some of the better designs incorporate a duplex strainer arrangement so that one strainer can be offline for cleaning and maintenance with no disruption in service. Some designs automatically change over and clean with no operator action required.