Winter 09 | Reprinted from LNGINDUSTRY.COM

U sers of cryogenic liquid pumps have many important criteria to evaluate when selecting a pump. For certain pump services one of the most important is the net positive suction head

required (NPSHR) value. There are many performance and economic benefits associated with low NPSH ability. To achieve this, pump designers often incorporate an inducer as the first stage hydraulic component. Over the years, inducer design has become more and more important as advances in technology have allowed for pumps with very small NPSHR values.

This article discusses how different inducer designs can affect NPSHR performance, and will offer some points to consider when evaluating pump NPSHR values.

Why is NPSHR so important?Users of submerged electric motor pumps (SEMP) are continually requesting pumps with lower NPSHR. This is due to the great benefits associated with low NPSHR for all types of SEMPs. For example, the NPSHR value of a loading pump installed within an LNG storage tank has a direct relationship to the effective usable volume of the tank. If the liquid is at saturated conditions, as is the case for most cryogenic storage tanks, then the minimum tank liquid level is defined by the NPSHR level of the pump. As such, a lower NPSHR will provide a greater usable volume of liquid, or allow a smaller tank to satisfy a fixed volume.

It’s all in the designMICHAEL CORDS,

EBARA INTERNATIONAL

CORP., USA, EXPLORES HOW NPSHR

PERFORMANCE CAN BE AFFECTED BY

DIFFERENT INDUCER DESIGNS.

Marine cargo pumps installed inside the storage tanks onboard LNG carriers are used to offload the LNG product. In this application, a low NPSHR is important economically because any product that cannot be offloaded cannot be sold. Ship designers are always looking for ways to decrease the remaining dead stock of liquid and have over the years pushed SEMP designers to decrease the NPSHR.

Even pot-mounted pumps, pumps installed inside their own containment vessel, can benefit from low NPSHR. Upstream equipment, such as a re-condenser, must be physically located at a defined elevation, with respect to the pump containment vessel, to provide sufficient net positive suction head available (NPSHA). A reduction in the NPSHR allows more freedom for placement of equipment and lower civil engineering costs. Because of these benefits, the NPSHR value at the rated flow rate of the pump is one of the criteria to be guaranteed by the pump manufacturer and it is one of the most important numbers on a manufacturer’s datasheet.

It should also be noted that NPSHR means different things to different pump designers. The generally accepted industry definition of NPSHR is the NPSH that results in a 3% loss of head. Many pump designers will quote an NPSHR value based on a 10%, 25% or even more than 50% loss of head. This allows a lower value to be shown on the datasheet. Therefore, it is important to always confirm the NPSHR definition being used by each pump designer when evaluating and comparing performance. The pump vendor can also be instructed to provide information per the definition desired. This will ensure that the pump can be properly selected and the pump designer can properly design for the pump performance required.

NPSHR and pump operationEvaluating NPSHR performance is an easy process, since a pump with a lower NPSHR value is better, right? Unfortunately, as with most engineering issues it is not as easy as it first seems. The reality of the service conditions and intended use play a large role in deciding how to evaluate the NPSHR performance of a pump. The NPSHR value varies as the flow rate of the pump changes. Pump designers use various inducer designs not only to minimise NPSHR, but to control this variation so as to optimise the suction performance of the pump to match its intended service.

Some examples will illustrate how pump operation and NPSH performance relate to one another. An in-tank pump, such as an LNG loading pump, is installed at the bottom of an LNG storage tank and is located within the discharge piping column of the tank. This type of pump tends to operate at, or near, its best efficiency point (BEP) or rated design flow rate, with little variation. In this situation a low NPSHR value at rated flow is important to maximise the usable volume of the tank. But there is also another point to consider. During startup the pump must first fill the tank’s discharge column, which can be up to 50 m in height. During this time, the pump is operating at maximum flow as the liquid has not yet reached the control valve. If the NPSHR value at maximum flow is too high, the pump may operate in a transient cavitation condition until sufficient backpressure is built from the column filling. In this case, NPSHR at maximum flow is just as important as NPSHR at the BEP flow rate (see Figure 1).

For a marine cargo pump it is important to strip the ship’s tank as low as possible, in order to minimise the dead stock of liquid. Thus, as the liquid in the tank reaches its minimum, operators will often reduce the flow rate of the pump to operate at a region of lower NPSHR. Here, the lowest NPSHR value, regardless of where along the flow range it occurs, is of prime importance.

Will the pump operate at a constant flow rate, or will process conditions dictate a range of flow rates? How often will low NPSHA conditions occur? These are some of the questions that must be answered to properly evaluate the NPSH performance required of a pump. Thus, a simple comparison of NPSHR values at rated flow is not sufficient to accurately judge the performance of one pump against another. A low number may look good in a pump vendor’s quote, but it may not tell the whole story, and it may not be what is required.

Inducer performance characteristicsSo how do inducers affect the performance of a pump with respect to NPSHR? Can different inducer designs influence NPSHR values at different flow rates? These are key questions that the pump designer must face. No single inducer design can do everything. So it is necessary to design an inducer that has the preferred characteristics that will optimise the overall performance of the pump. What is good for one type of pump may not be good for another.

Inducers are basically axial flow impellers intended to lift the liquid into the eye of the proper first stage impeller. This improves the suction conditions at the first stage impeller and allows the pump to operate at lower NPSH conditions. While fan type designs were once employed, today inducers are generally of a spiral/helical type configuration. The inducer vanes are at a low angle of incidence to the inlet liquid stream. By varying the vane angles, vane pitch, diameter, number of

Figure 2. Various inducers matched with their suction impellers at the prototype stage (photo courtesy of Ebara Corp.).

Figure 1. Comparison of NPSHR levels at BEP and maximum flow rates.

Reprinted from LNGINDUSTRY.COM | Winter 09

vanes and other geometries, the pump designer can change not only the magnitude of the NPSHR value, but also how the NPSHR value changes across the operating flow range.

Variation in inducer vane pitch can illustrate just how effective the design of an inducer can be for optimising pump performance. Vane pitch is defined as the axial length travelled by a vane, per degree of vane rotation. In other words, for a given length, a high pitch inducer will have more wraps of each vane than a low pitch inducer. This pitch can be constant or it can be varied along the axial length of the inducer.

Variable pitch and constant pitch inducers have different NPSHR characteristics. A variable pitch inducer can generally achieve a lower NPSHR at the BEP of the pump. It also has superior performance at lower flow rates. However, at flow rates above the BEP, its NPSHR values increase greatly. At these higher flow rates, a constant pitch inducer will have better NPSHR performance. For pumps where operation at lower flow rates is necessary, as is the case for tank stripping, a variable pitch inducer is preferred. For pumps required to operate at high flow rates, a constant pitch inducer is superior. The two designs may have the same NPSHR value at the rated flow rate, but knowing how the pump will be operated in service will dictate which design is best.

Another design consideration is the specific speed of the inducer. High specific speed inducers were first developed in the rocket turbo pump engine industry and were quickly employed by pump designers for their low NPSHR characteristics. They have found a place in SEMPs and have advanced the technology of pump performance.

However, one drawback of high specific speed inducers is their narrow operating range; the higher the specific speed, the narrower the stable window of operation. Again, it is the intended service operation of the pump that will dictate the inducer design. A pump that operates mostly at its rated flow rate will benefit greatly from a high specific speed inducer designed to match this flow rate. For pumps that require a greater operating range, such an inducer may not be ideal.

Engineering an inducer SEMPs are custom, engineered to order machines. The design of an inducer is an important step in the overall design of the pump. The inducer is an integral component with the first stage impeller. For example, it is a key design consideration that the inducer outlet flow be matched with the impeller inlet geometry. This not only affects the NPSHR, but can affect the overall efficiency of the pump. In fact, it is now common for the first stage impeller to be a dedicated suction impeller, designed in harmony with the inducer (see Figure 2). Experience with various inducer and impeller designs is necessary to optimise the overall performance of the pump.

Today, computational fluid dynamics (CFD) also plays a key role in the design of custom inducers (see Figure 3). The ability to accurately predict not only the NPSHR performance, but also the overall pump curve shape as it’s affected by the inducer design, is enhanced by such modelling tools.

In order to ensure that the best possible NPSH performance is achieved, it is necessary to identify the true and complete operating conditions of the pump. It is here that discussion with an experienced pump designer will pay dividends in the final pump performance. A complete understanding of how

the pump will be operated, and what is actually required over the entire flow range, will allow the pump designer to optimise the performance of the inducer.

ConclusionLow NPSHR is a valuable performance characteristic of SEMPs. There are several performance and economic benefits to be had in which inducers play a very important role. As such, the design of an inducer is a key aspect of the overall pump design. Experienced pump designers are necessary to optimise overall pump performance.

It is also important to discuss the service conditions and operating characteristics with the pump designer. This information will help to ensure a correct inducer design is implemented for overall performance benefit. The best inducer will optimise all performance conditions. The best inducer for the job at hand is more than just one number on the datasheet.

References1. RUSH, S. and KAUPERT, K., ‘Submerging Solutions’, LNG Industry,

pp.77 - 80, Summer 2007.

2. KAUPERT, K. and KAMIO, K., ‘Advancing Suction Performance in Hydrocarbon Liquids’, ASME Fluids Engineering Division Summer Meeting, FEDSM2009-78050, 2009.

3. STEPANOFF, A., ‘Centrifugal and Axial Flow Pumps 2nd Edition’, Krieger Publishing, pp.144 - 151, 1957.

4. WATANABE, H. and ICHIKI, I., ‘Development of Cryogenic Pump Inducer using Inverse Design Method and CFD’, Ebara Engineering Review Vol. 221, pp.3 - 11, October 2008.

Figure 3. Computational fluid dynamics (CFD) is now a common tool used for inducer design (photo courtesy of Ebara Corp.).

Winter 09 | Reprinted from LNGINDUSTRY.COM