How to Optimize Centrifugal Pump Operation, Part 2

Incorrect piping can result in hydraulic instability and cavitation in a pump system, among other problems. To prevent cavitation, the focus should be on the suction piping and the suction system design. High levels of noise and vibration could be caused by cavitation, internal recirculation and air entrainment—damaging conditions for seals and bearings.

Pump Recycle Line

When a centrifugal pump—particularly a small pump—must operate at different operating points, a recycle line can return some of the pumped liquid. The pump can continue to operate efficiently and reliably at the best efficiency point (BEP). Recycling liquid wastes some power, but the amount can be insignificant for a small pump. A recycled liquid should be routed back to the suction source instead of the suction line. Suction line connections can cause turbulences at the pump suction, resulting in operational problems and even damage. The recycled liquid should be routed back to the other side of suction tank instead of the pump suction intake point. Often, a suitable baffling plate arrangement or other similar designs can ensure recycling without turbulence.

Parallel Operation

Multiple smaller pumps in parallel operation are often necessary when a single large pump is not available for some high-flow applications. For example, some pump manufacturers may not supply a pump frame large enough for a high-capacity pumping unit. Some services require an operating flow range so wide that a single pump cannot function economically. For these services where the power rating is high, the recycling or operation of a pump far from its BEP can result in significant power waste and reliability issues. When pumps operate in parallel, each pump produces a lower flow rate compared with that pump when operated alone. When two identical pumps operate in parallel, the total flow is less than two times each pump’s flow. Despite particular application needs, parallel operation is often employed as a last solution. In many cases, for example, two pumps in parallel operation are preferable to three or more, if possible. The parallel operation of pumps can be a risky and unstable operation. Pumps for parallel operation require careful selection and delicate operation and monitoring. Each pump curve needs to be similar—within 2 to 3 percent of tolerances. The combined pump curve must remain relatively flat.

Pump Piping

A poor piping design will easily translate into high pump vibration, bearing problems, seal issues, premature failure of pump components or catastrophic failures. Suction piping is particularly important because the liquid should arrive at the pump impeller eye with the right pressure and temperature, among other operating conditions. Smooth, uniform flow will decrease the risk of cavitation and lead to reliable pump operation. Piping and passage diameter has a significant effect on head. As a rough estimate, the pressure loss from friction would be inversely proportional to the fifth power of the pipe diameter. For example, a 10-percent increase in the pipe diameter could result in about 40-percent reduction in the head loss. In the same way, an approximately 20-percent increase in the pipe diameter could result in a 60-percent reduction in the head loss. In other words, the frictional head loss would be less than 40 percent of the head loss at the original diameter. The importance of net positive suction head (NPSH) in pumping applications makes pump suction piping design a significant factor. The suction piping should be as simple and straight as possible, with minimum overall length. A centrifugal pump should usually be provided with a straight run of about six to 11 times the suction piping diameter to avoid turbulences. A temporary suction strainer is generally required, but a permanent suction strainer is usually discouraged.

Reducing NPSHR

Piping and process engineers sometimes attempt to reduce the NPSH required (NPSHR) compared with an increase in the NPSH available (NPSHA). Reducing NPSHR is a very difficult and costly process with few options because NPSHR is a function of the pump design and the pump speed. Impeller eye and overall pump size are important considerations for pump design and selection. A pump with a larger impeller eye can offer a smaller NPSHR. However, a larger impeller can result in some operational and hydrodynamic issues, such as recycling problems. Slower pumps usually require less NPSH, while faster pumps require more. Pumps equipped with specially designed large-eye impellers could result in high recirculation issues, lowering efficiency and reliability. Some low-NPSHR pump designs feature such low speeds that the overall efficiency is not economical for the application. These low-speed pumps also suffer from a low reliability record. Large, high-pressure pumps suffer from physical site constraints, such as the pump location and the suction vessel/tank arrangement, that prevent end users from finding a pump with an NPSHR to fit the limits. In many renovation projects, the site layout cannot be changed, but the site still requires a large, high-pressure pump. In these cases, a booster pump should be used. A booster pump is a smaller, low-speed pump with a low NPSHR. The booster pump should offer the same flow rate as the main pump. A booster pump is usually installed in a close distance upstream of the main pump.

Determining the Cause of Vibrations

Low flow—usually lower than 50 percent of BEP flow—results in several hydrodynamic problems, including noise and vibration from cavitation, internal recirculation and air entrainment. Some pumps can resist suction recirculation instabilities at very low flows, sometimes as low as 35 percent of BEP flow. For other pumps, the suction recirculation could be seen at about 75 percent of BEP flow. The suction recirculation can result in some damages and pitting, usually at around halfway along the pump impeller vanes. Discharge recirculation is a hydrodynamic instability that can also be seen at a low flow. This recirculation can occur from improper clearances at the discharge side of the impeller or impeller casing. This also results in pitting and other damage. Vapor bubbles in the liquid stream can result in instability and vibration. The cavitation usually damages the eye of the impeller. The noise and vibration from cavitation may be similar to other malfunctions, but a pump inspection at the location of pitting and damages on the pump impeller can usually reveal the root cause. Gas entrainment is commonly seen when pumping liquids close to their boiling point or when complex suction piping encourages turbulence to occur.