Process Puzzler: Preclude Pump Problems

Achieving adequate performance requires multiple measures

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Catastrophic pump failures at the tank farm are plaguing the commissioning of our new plant. The tank farm has an ordinary centrifugal pump (CP) as well as magnetic-drive CPs and a gear pump (see figure). The mag-drive pump started failing after the first few days. We got some warning from a pressure switch low (PSL) that flashed for a couple of minutes. We switched to a spare and it did the same thing after a day. The gear pump seemed to be operating fine, even after a few days — but a reading on an infrared gun indicated the casing was hot; the downstream flow meter showed a dropping flow rate. An operator loosened the packing on the pump while I was away at lunch and it now runs fine. The water pump is showing the same symptoms as the mag-drive pump but hasn't failed yet during the first week of commissioning. Are we out of the woods?

I have the following observations and comments. Regarding the first pump, assuming it is a metal magnetic-coupled pump: the flow rate may be too large, causing the pump to run beyond its operating range. This may be caused by the bypass if the orifice pressure drop is too small, or the flow rate to the consumer is too large. It might be advisable to check the flow rates, for example, using a clamp-on ultrasonic flow meter. At a high flow rate, a magnetic-coupled pump may encounter too high radial forces and a loss of lubrication in its sleeve bearings, causing the failure, before motor protection can cut in.

Now, consider the water pump. This pump also may be running at too high a flow rate, not yet triggering motor protection, since flow seems not to be restricted by a control valve, etc. This flow rate may also be checked using a clamp-on flow meter.

I assume that suction-side strainers have been checked for both pumps and they are not (partially) blocked. I also assume that PSLs for these pumps are set to pressures higher than the geodetic height, i.e., the full level of the tank over pump.

These failures may be aggravated by a high liquid level in the tanks, which reduces the pressure drop the pump has to overcome. At high level, the pumps may seem to be oversized. Using power consumption measurements to check operating points is another way to determine the pumps' flow rates, but this lacks precision. My advice is to use flow controls to restrict the pumps' operating ranges.
Dr. Walter Schicketanz, consultant
Ing. Pumpenfachingenieur GmbH, Rosenheim, Germany

If you read many installation manuals you will find that a medium strainer (40–60 U.S. mesh) often is used to catch debris during startup from suction lines. A tank farm application might consider a 100-mesh self-cleaning suction strainer; above 30 mesh the strainer basket must be reinforced. So, let's consider the details.

It sounds like you need to replace the elements in the strainer with coarser ones. Usually, this is a U.S. mesh 3–8 (with holes: 7.42 mm and 2.54 mm, respectively) but to be more exact you should refer to the pump cut sheet; I've seen 500 mesh used in pump suctions. In these cases, opt for a coarse strainer upstream and have your controls watch for flow restriction or pressure drop, say, by an increase in control valve position.

The pressure drop can be estimated by the following equation:

∆P = (Q/Cv)2×SG×C1×C2

where Q is flow rate in gal/min, Cv is a vendor-supplied flow value for the filter, SP is specific gravity, C1 is the mesh correction factor (C1 = 1 for 8 mesh and below) and C2 accounts for the % of area clogged. Vendors usually use C2 = 2 for 50% clogged. A rule-of-thumb, if you don't have any other data, is to take strainer pressure drop as 1 psi at the (maximum) design flow.

For the mesh, look for the maximum solids (assumed diameter) that the manufacturer allows. In Europe, suction strainers are specified as requiring a hole diameter of 67% of the pump manufacturer's maximum solids size and a surface area 3–5 times the pipe cross-sectional area. Don't run a pump without a strainer because this violates your warranty and could put you out of business; I've seen zebra mussels and even a big chunk of a pallet go through suctions and demolish impellers.

As for the operator, he pulled a fast one. The gear pump is operating without a basket. Discourage this practice by locking down the strainers as you would lock-open a valve. Gear pumps have even tighter constraints than centrifugal pumps: a gear pump won't survive a pallet. Consult the manufacturer for specific details with equipment like gear pumps and wet gas compressors; one compressor manufacturer listed 11 mm as the minimum impeller clearance but this assumes there is a working mesh pad in the upstream knockout drum — always consult the manufacturer. I guessed 67% of 11 mm for the compressor and later was proved correct.

Some pumps are even more delicate. An air diaphragm pump usually requires a 40–60 mesh strainer because of the piston check valve.

Now, let's consider some of the other reliability problems. First, you don't need double seals for a water pump. Single seals, packing or both in tandem will suffice. Because gear pumps are not pressure-variable, you should use a flow alarm to catch problems such as missing teeth and a blocked valve. Magnetic-drive and seal-less pumps pose a particular problem because of tight clearances. As a general rule, a PSL is not an effective tool even with double-seal centrifugal pumps used as the baseline in the refinery and chemical industry. Use a flow meter or switch, not a pressure switch, to detect a problem with a centrifugal pump.

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