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

The size of the suction pipe must have an adequate diameter to keep friction losses down and velocities in an acceptable range. I always recommend to keep liquid velocities on the suction side below 2 meters per second (6.6 feet per second) at the maximum, and one meter per second (3.3 feet per second) is better. Just as the suction pipe size must be adequate, so must the strainer size.

Flow Coefficient Formula

Strainers should be engineered and selected to keep the pressure drop down to an acceptable low and safe level. The industry rates strainers using “CV”—known as the flow coefficient. When selecting strainers, I suggest you work with a knowledgeable expert, because the selection process can be tricky for the uninitiated. Most strainer resistance coefficients are based on water and a given mesh size for the strainer screen. You may need to correct for viscosity and a different mesh depending on your fluid properties. You will also need to decide the capacity ratio known as open area ratio (OAR). The OAR will indicate how long you can operate the strainer before it will require cleaning or replacement.

In a good system design, the pump should be positioned close to the suction source, but there should also be at least 5 to 10 pipe diameters worth of straight, unobstructed piping connecting to the pump. Never connect any components such as an elbow, reducer, valve or strainer within the final run of pipework. A clogged strainer will present a turbulent flow profile to the pump suction. If you connect an elbow directly to the pump flange, the fluid will be forced toward the outside of the elbow and will not be directed into the center of the impeller. Most pumps are designed for fluid to be evenly loaded at the center of the impeller. Otherwise, the imbalance creates stress on the pump’s bearings and seals that leads to wear and premature failure.

Acquiesce on Strainers & Filters on Suction Side

It is noted and acknowledged that many processes are purposely designed to have strainers on the suction side of the pump. Some processes actually rely on the reduced pressure (vacuum) created in the process. An example would be a filter press. Note these systems and pumps are designed and instrumented for the additional stress.

Conclusion

Every rule usually has exceptions, but from the perspective of an industry best practice, I suggest thinking long and hard before the placement of strainers on the suction side of the pump—and you “exceptions” already know who you are.

Many pumps are designed to handle some amount and size of solids. The manufacturer can or will advise what size solids the pump will handle. It is typically a better idea to place the strainer on the discharge side of the pump if possible. If you must place the strainer on the suction side for a valid design reason, then take the proper steps to design the system to prevent pump issues caused by clogging.

For more than 48 years, I have watched in amazement as operators destroyed their pumps by starvation.

To read other articles in the 'Common Pumping Mistakes' column, go here.

Strainers & Filters

One person’s strainer may actually be another’s filter. Some people use the terms interchangeably, but typically the term “strainer” is actually a subset of “filter.” I have used the word strainer throughout the article, but note it could also be a filter.

A strainer removes solids from a solution using a purely mechanical and particle size-based sieving process.

If the particle is larger than the size of the holes, it will not pass. If the particle is smaller than the holes, it will pass.

Filter is really a general term. It describes a device that removes solids from a fluid that is passing through the filter media. The solid could be removed by straining, but it can also be removed by other methods.

Technically, some people define a filter at less than 75 microns and a strainer at 75 microns and larger. In my experience, the colloquial expression is to use the word “strainer” if the device has a metal screen, regardless of the mesh size. Otherwise, the word “filter” is used if the device has bags or cartridges, regardless of the micron rating. You may have other definitions in your facility.

Pages