The same things apply to package systems, where the focus tends to be on getting as much equipment into as small a space as possible with little regard for piping design.

Rule No. 5

Remove the potential for vortices or air entrainment in the suction source. Eliminate all piping configurations that might introduce an air pocket into the suction side of the pump. These air pockets can create turbulence and air entrainment, which can result in impeller damage and vibration.

If a pump is taking its suction from a sump or tank, the formation of vortices can draw air into the suction line. This usually can be prevented by providing a sufficient level of liquid over the suction opening. A bell-mouth design on the opening will reduce the amount of submergence necessary. This submergence is completely independent of the net positive suction head required by the pump.

Take great care in the design of a sump to ensure that any liquid emptying into the sump does so in such a manner that air entrained in the inflow does not pass into the suction opening. Any problem of this nature may require a change in the relative positions of the inflow and outlet if the sump is large enough or necessitates the use of baffles.

Rule No. 6

Arrange the piping so it imposes no strain on the pump casing. Accurately align piping flanges before the bolts are tightened and independently support all piping, valves and associated fittings. Building the piping from the pump to the system can help reduce pipe strain. Any stresses imposed on the pump casing by the piping decrease the probability of satisfactory reliability and performance.

 

Laser pipe-alignment tools offer significant benefits. Mounted on the pump or the piping, such tools can provide an effective way to prevent pipe strain during initial installation, which is exactly the right time to stop it.

The effects of pipe strain in a conventional process pump can also be transmitted to the shaft coupling. With the accuracy of laser alignment technology, pipe strain can be measured and should be integrated into the shaft alignment program. By loosening the pipe connection after a correct shaft alignment has been performed, any movement at the coupling will indicate the existence of pipe strain and can therefore be corrected.

However, there is an exception to this rule. The American Petroleum Institute (API) 610 Pump Specification identifies a maximum level of forces and moments that may be imposed on the pump flanges. So, any pump being sold in the petroleum or a related industry must comply with that specification. As a consequence, all API pumps are of a much more robust and heavier design than their ANSI size equivalents.

In high-temperature applications, some piping misalignment is inevitable because of thermal growth during the operating cycle. So, thermal expansion joints often are used to avoid transmitting any piping strains to the pump. However, if the end of the expansion joint closest to the pump is not anchored securely, the pipe strain is passed through to the pump, thus defeating the objective of the expansion joint.

Rule No. 7

When operating at consistently low flows, use a constant bypass line. If prolonged periods of low flow operation are expected due to variable system demand, install a bypass valve to regulate the flow through the pump. It may also prevent an excessive number of pump stops and starts, which is particularly important with larger motor drivers.

Direct the excess flow back to the suction source of the pump -- not to the suction of the pump -- otherwise, excessive line turbulence may result.

Don't disregard piping

Piping design is one area in which basic principles are frequently ignored, resulting in problems such as hydraulic instabilities in the impeller, which translate into additional shaft loading, higher vibration levels and premature failure of the seal or bearings. Because many factors can contribute to pump vibration and seal and bearing failure, the trouble rarely is traced to incorrect piping.

Some may argue that since many pumps are piped incorrectly, yet operate satisfactorily, piping procedure is not important. However, that doesn't make questionable piping practice correct; it merely makes getting by with it a matter of luck.

Any piping mistakes on the discharge side of a pump frequently can be accommodated by increasing the performance of that pump. Problems on the suction side, however, can cause repetitive failures, which may never be traced back to that area and could continue undetected for many years to come.

Ross Mackay has more than 40 years of experience in the pump industry, and now specializes in consulting and training to improve the economics and reliability of pump installations. Call him at (800) 465-6260.