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Peculiar Pump Types for Specific Applications
Vinod Shenoy It is imperative that a precise pump type is chosen for a given application at the front end or basic engineering stage of plant design. This will not only help in reducing the overall lifecycle cost of the plant but also result in reducing the downtime and thereby improving the reliability factor.

The Process Industry is dominated by variety of pumps, most of which are conventional centrifugal pumps. Some process requirements often demand rather unconventional types. In some extreme cases, the driving factor for pump selection could be completely different such as space limitation.

This article intends to capture a few of such types as follows:
1. Low Flow- High Head Duty: As such, there is no specific quantification to define the exact boundaries of Low Flow -High Head. However, typically, capacities between 1 & 35 M3/Hr and head up-to 300 MLC (occasionally until 1600 MLC) fall under this category.

Conventional centrifugal pumps cannot handle low flow high head duties because of suction recirculation and temperature rise. The minimum continuous flow is usually set by the pump manufacturer to avoid suction recirculation. Suction recirculation results in increased vibration and imparts continuous axial movement to the shaft, decreasing the life of the bearings and mechanical seals. Also, temperature rise through a pump determines the minimum continuous thermal flow rate.

Let us look at a few peculiar pump types, which have been around for a while and have found their way into specific applications.
a. Partial Emission Pumps: The type of pump often used for such low flow high head requirements is a single port diffuser pump with a straight vane impeller also known as Partial Emission pump. The special hydraulic design inherently prevents back-flow at low capacity, while achieving a stable curve with extremely low NPSHr values without Peculiar Pump Types for Specific Applications the need of an inducer. Open impeller and concentric volute design substantially reduce radial and axial thrusts on shaft and bearings, regardless the operating capacity and suction pressure. In low flow-high head duties, this design gives higher reliability when compared to traditional volute pumps by reducing vibrations and shaft deflections. The impeller, of the open type, star design without wearing rings, rotates within the casing with relatively wide clearances thus allowing safe reliable operation and preventing seizures due to thermal expansions at high temperatures. For the same reasons, a certain amount of particles in the pumped liquid can also be tolerated.
Typically, these pumps can cater to capacities from 1 to 35 m/h and upto 300 m total head with a single impeller. Although, these pumps do not substitute the high speed integrally geared pumps they fill the void left by the conventional centrifugal pumps.

b. Pitot Tube Pumps: In general, pitot tube pumps are used for low flow and relatively high head applications. These pumps can achieve about 1600 m head based on water at flows upto 90 m/h. Hence, this type can be an interesting economical alternative to the high speed integrally geared pumps. However, it may be noted that these pumps require more floor space than a conventional centrifugal pump and a high-speed integrally geared pump. The pitot tube pump is a variation of the roto-dynamic pump design. It uses a closed casing that is attached to and rotates with the impeller (cover) while a stationary pitot tube captures the discharge flow. The inlet of the pitot tube is positioned near the maximum inner diameter of the casing. The fluid enters the impeller (cover) along the axis of rotation and picks up momentum as it passes through the radial vanes of the impeller and into the casing.

The liquid in the casing maintains an angular velocity slightly less than that of the casing. It then impacts the inlet orifice of the pitot tube near the periphery of the rotating casing. The fluid is then discharged through the internal passageway of the pitot tube and out of the pump. This results in a continuous, high-pressure flow of liquid from the pump discharge. The pump generates a pulsation free flow and has a stable NPSHr curve. The total head developed by this type pump is the sum of both the static pressure head and the velocity head. This sum is equal to approximately 1.6 times the head produced by a conventional roto-dynamic pump of the same impeller size and speed.

2. Side Channel Pumps: The side channel pump was invented in Germany, about 100 years ago. Today, the side channel pump has become indispensable in applications such as condensate recovery, LPG, CO2, refrigerant transfer, etc. with capacities upto 35 m/h and 350 m head. There are some manufacturers who offer these pumps with a magnetic coupling as well. When started, the pump begins to work as a displacement pump due to its self-priming characteristics, but once the side channels are completely filled with liquid, the pump works as a centrifugal pump.

Due to the centrifugal effect, the fluid is collected in the outer region of the side channel as well as the wheel cells, where it forms the liquid ring responsible for its self-priming characteristic. On the inside, the gas is concentrated. Side channel pump can also operate as a Vacuum pump during a limited amount of time, before the auxiliary liquid increasingly heats up and finally evaporates.

Salient features of side channel pumps:
i) This pump type is capable of handling large gas flows with the liquid. In certain processes, it is a requirement that a pump can, at least temporarily, evacuate gases and vapours from the process without the liquid flow being interrupted. Normal radial centrifugal pumps have a very limited capability of transporting gas within the medium. Even at relatively low gas contents, the liquid flow can immediately interrupt or damage the pump bearings due to dry running.

ii) The pressure build-up caused by the sum of the individual pulses during the liquid movement creates an internal multi-stage-effect, even with just one impeller. Lower potential pump speeds become possible due to higher pump heads and therefore help to reduce the pump s NPSH value against high-speed pumps. A slower pump speed has the additional advantages that mechanical loads, par t load performance, and noise can be controlled better and the pumps  expected service life is generally increased.

iii) The side channel pump with variable vapour pressure is significantly less vulnerable to cavitation compared to radial centrifugal pumps because of the lower specific speed. These pumps can also handle media close to its boiling temperature (e.g., LPG).

Some of the distinguishing characteristics of Side Channel Pump Performance are as follows (refer attached curve):
• The curves of these pumps are different from centrifugal pumps and are more likely to be mistaken for axial flow pumps. They have their highest power consumption at their highest discharge pressure and their lowest flow rate.
• Their steep Capacity /He a d (Q/H) curves are ideal for pressure-dependent system regulation.
• Unlike centrifugal pumps, the star shaped impeller cannot be trimmed to adjust to the required duty conditions. Pitot Tube Pump
• There is no definite shut off limit for these pumps and thus pressure relieving devices are absolutely essential. A Pressure transmitter is optional whereas a pressure relief valve has to be considered for safe operation.

3. API 610 Between Bearing Pump (BB2*) in Vertical side-side Execution: BB2 configuration as defined in API 610 is very common in the refinery and petrochemical industry which means a between bearing horizontal pump with top top suction and discharge configuration.

However, in the oil and gas industry, offshore installations put a huge price tag on the weight and dimensions of any equipment. The adjoining picture gives a clear idea of the arrangement for this type of pump in vertical side-side configuration. This arrangement occupies considerably lower floor space and thus, a popular choice for a variety of process applications in the offshore industry. Globally, there are not many players who manufacture such special pumps.

Some manufacturers can also offer BB2 pumps with Horizontal side-side nozzle configuration. Again, whether to choose such pumps is purely layout driven as there are not many known manufacturers with such models. Also, their casting patterns are fixed which means that customization with respect to suction and discharge nozzles orientation may not be possible. Therefore, while opting for a BB2 type pump having Horizontal side-side nozzle configuration, compliance to layout requirements must be obtained from the Design team of the manufacturer. (*Between bearing pumps with one or two stages classified by American Petroleum Institute.)