Prevent Suction Piping Problems

Aug. 13, 2018
Follow best practices when designing pump systems

Piping issues can directly affect a pump’s performance and life. Poorly designed suction piping can result in pump damage and even failure. Quite bluntly, there’s no excuse for substandard piping design.

Numerous guidelines and mandates in the technical literature, textbooks, manuals, codes, specifications, etc., call for short and simple suction piping. Yet, some engineers and designers still treat such dictates only as preferences. They install pumps far from suction sources and design long and complex suction piping systems. I personally can attest that many design teams don’t heed the guidelines for suction piping. They offer excuses such as there’s no space near the suction vessel (tank or drum) or it’s more convenient to install pumps near downstream equipment.

As a result, cavitation and other suction-related problems such as turbulence and air entrainment cripple pumping systems in many applications. Root-cause analysis of pump failures often points to long suction piping systems as the culprit. The solution to avoiding future failures usually is redesigning the suction piping to be as short, simple and straight as possible.

You should consider pump location and suction piping at the layout stage. It’s simply wrong to fix the location of every vessel, drum or tank and leave pump locations for later. You also should anticipate the addition of small pumps in due course; for such cases, provide spare space around vessels, tanks or other equipment to accommodate these pumps right at the layout stage. In addition, make your best efforts to place any pumps close to the suction source.

Always explore any possible option to install pumps closer (even if only by 1 m) to the suction source. Pump textbooks and nearly all pump catalogues and manuals clearly note that suction piping should be as short, simple and straight as possible. Unfortunately, some design teams opt for the easiest design rather than correct one (as per guidelines).

The Basics

For any suction piping longer than a few meters, ensure that you provide enough net positive suction head (NPSH) margin, i.e., NPSHA - NPSHR, for all potential operating points on the performance curve of the pump from shutoff to near the end of the curve. An adequate margin particularly is needed at or near the end of the curve where NPSHR is high and NPSHA is low (because of high flowrate).

Different guidelines offer various recommendations for margin, for instance, 1 m, 1.5 m or 2 m, depending on the criticality of the application, pump details, suction energy, sensitivity of pumps, potential damage due to cavitation, etc. A good recommendation is a minimum NPSH margin of 2 m for the commonly used operating range (say, 70–120% of the rated point) and a minimum NPSH margin of 1 m for the end of the curve to prevent risk of cavitation when the pump operates, even temporarily, at the far-right side of rated point.

Cavitation can cause a wide range of damaging and disturbing effects such as suction pressure pulsations, erosion damage, increased vibration, noise, etc. Check the margin for the worse possible operating cases, for instance, when the suction source is at its minimum head or liquid level, friction in suction piping is at its maximum, etc.

These guidelines may necessitate an increase in the suction piping size. For relatively long and complex suction piping, it’s common to see suction piping up to four sizes larger than the size of the pump’s suction nozzle; for instance, a 125-mm pump suction nozzle may require 250-mm suction piping (for a relatively long run). If such a size increase isn’t viable, consider installing a drum or small tank near the pump to act as the suction source for it.

Connect the pump nozzle to an appropriate length of straight pipe, per the pump manufacturer’s guidelines. As a very rough indication, the minimum length of straight pipe needed between an elbow (or any major fitting) and the pump suction nozzle is 4–12 times the diameter of the suction piping. For some high suction energy pumps, this straight length should be up to 15 times the diameter; for commonly used small pumps, which usually are low suction energy units, this required straight length is somewhere between three and six times the diameter of suction piping.

The straight-run pipe gives a uniform velocity across the suction pipe diameter at the pump inlet. Keeping the suction piping short ensures that pressure drop is as low as possible; this directly affects the NPSH margin. These two factors are important for achieving optimal suction and trouble-free pump operation.

For any suction piping not conforming to short and simple guidelines, check with the pump manufacturer. It’s common to ask the vendor to review suction piping and make comments on the performance, functionality, reliability and all guarantees of the pump with that suction piping. The bottom line is that the pump manufacturer should confirm that the pump isn’t affected by that suction piping. Remember that pump guarantees often are limited to two or three years, so correct suction-piping design is a better way to ensure proper long-term performance.

Turbulence And Air Entrapment

Sizing of suction piping isn’t the only area requiring attention. Also, seriously evaluate route, layout and configuration. Suction flow disturbances, such as swirl, sudden variations in velocity or imbalance in the distribution of velocities and pressures, can harm a pump and its performance and reliability. For any suction piping a bit longer than usual or not straight and simple, ensure that adverse effects such as turbulence, disturbances, air entrainment, etc., won’t affect the pump set.

Minimize the number of elbows in the proposed suction piping; numerous elbows might present swirl, disturbances and other damaging effects to suction flow and, consequently, to the pump. Eliminate any elbow mounted close to the inlet nozzle of pump. Especially avoid two elbows at right angles because they can produce sustained damaging swirls. There have been cases where a swirl introduced by two elbows in the suction caused high vibration of the pump and subsequent damage to it.

Another type of damaging flow pattern to a pump results from swirling liquid that has traversed several directions in various planes; therefore, avoid complex suction piping routes with multiple directional changes. Usually, the higher the suction energy and specific speed of a pump, in addition to the lower the NPSH margin, the more sensitive a pump is to suction conditions.

Also, eliminate the potential for air entrapment in the suction piping. One of the sources of air or gas entrainment is the suction tank or vessel. You must maintain adequate levels in the suction source (drum, vessel or tank) to keep vortices from forming and causing air/gas entrapment. In addition, ensure there’s no air/gas pocket. Particularly avoid high pockets in suction piping; these can trap air or gas. Suction flanges or any connection with potential leaks can be a source of air entrainment; so, minimize the use of flanged connections and eschew threaded ones. Check that all piping and fitting connections are tight in suction vacuum conditions to prevent air from getting into the pump.

Velocity in the suction piping should rise as the liquid moves to the suction nozzle of the pump; this speed increase usually comes from reducers. The suction piping design should provide smooth transitions when changing pipe sizes. Often, two or three reducers are used (usually back to back) to decrease a large size of suction piping to the size of the pump’s suction nozzle. Pumps should have an uninterrupted flow into the suction nozzle. Generally, install eccentric reducers with the flat side on top to avoid the potential of forming an air/gas pocket.

Treat isolation valves, strainers and other devices used on the suction side of a pump with great care. Eliminate them if possible. I have seen many unnecessary isolation valves or permanent strainers on the suction of pumps; these cause more harm than good. If you absolutely require a valve, strainer, etc., size and locate any necessary device to minimize disturbances of the suction flow. Install these flow-disturbing items relatively far from the pump to let the provided straight length of piping smooth and normalize the liquid’s flow pattern.

AMIN ALMASI is a mechanical consultant based in Sydney, Australia. Email him at [email protected].

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