Deciding between placing this equipment on the suction or discharge side of the pump.
by Jim Elsey
January 22, 2019

System designers frequently place strainers on the suction side of a pump. This practice, ostensibly based on good intentions, is rarely a good idea and will create serious issues for the pump from the aspect of reduced and turbulent flow, inadequate net positive suction head available (NPSHA) and eventually blocked flow.

The consequential reduction in NPSHA will create deleterious effects for the pump and the downstream system.

These effects will always manifest as negative and expensive issues. Due to vibrations from cavitation and poor seal face lubrication from mixed phase fluids, the mechanical seals, like canaries in the coal mine, are typically the first component to fail, followed shortly by the bearings. These events create reductions in efficiency and add to unscheduled downtime, which all add to the total cost of ownership.

There are several acceptable methods and alternative designs to avoid these issues. There are also situations where placing the strainer on the suction side is the right thing to do, but it must be done correctly.

Background

Normally the reasons for placing the strainer on the suction side of a pump seem like good ones, as it is prudent to protect the pump and the other ancillary components in the system.

This preventative design approach can work to preclude or mitigate fouling of heat exchangers, valve blockages and other component issues with small operating clearances and annulus voids, to name a few. In the case of some positive displacement pump types such as gear and screw pumps, it is imperative.

General Comments

There are several issues with placing the strainer in the suction line.

The first issue is that the strainer will inevitably clog. The clogging will reduce, and at some rate of closure, completely shut off flow to the pump suction, creating serious damage to the pump. The blockage also creates issues with the system and downstream components that rely on the flow provided by the pump. Example: another pump in series with the first pump.

The second issue is a marked reduction in NPSHA. The formula for NPSHA is shown below for reference. Every pump has a requirement for a given amount of suction energy that is referred to as net positive suction head required (NPSHr). The specific requirement is determined by the manufacturer and is published on their performance curves. The NPSHr data is determined by empirical means.

If you have read some of my previous articles, you already know that the suction side of the system must provide energy to deliver the liquid to the pump. The pump does not reach out and pull the fluid into the impeller. The pump does not suck the fluid into itself, as fluids do not have tensile strength. The suction system must provide a level of NPSHA. The level of NPSHA is either calculated or measured by the owner or operator of the system. It is imperative that there is more NPSHA than NPSHr. This difference is referred to as the margin. The Hydraulic Institute (HI) and American National Standards Institute (ANSI) have a published standard that covers this subject—ANSI/HI 9.6.1-2012.

The issue with insufficient margin is cavitation and the subsequent deleterious effects from cavitation, which include material damage (usually manifests on the impeller), vibration, mechanical seal and bearing damage, loss of efficiency and loss of flow or partial flow.

The formula for NPSHA

Remember that we are calculating NPSHA, so we do not need to include velocity head. Velocity head would be included if we were measuring NPSHA.

Equation 1

NPSHA = ha – hvpa + or - hst – hf

Where:
ha = the absolute pressure. Absolute pressure as measured in feet of head of the liquid being pumped at the surface of the liquid.

hvpa = the vapor pressure. The head in feet corresponding to the vapor pressure of the liquid at the temperature being pumped.

hst = the static head of the liquid over the pump centerline for a flooded suction in feet (positive value for flooded suction)

hst = the static head of the liquid below the pump centerline for a lift situation in feet (negative value for lift situations)

hf = the total friction loss in feet of head for the suction side system

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