In this post, I want to share to you general selection guidelines of rotary pumps.

Rotary pumps are positive displacement pumps. However, they are different from reciprocating pumps because they have relatively steady and non-pulsating flow. Rotary pumps are generally selected to handle very high viscosity fluid or if the flow rate is too low to be handled economically by other pumps.

There are several common applications of rotary pumps:

• Transferring gasoline, fuel oil, diesel fuel into tanks or day tanks
• Supplying fuel oil to burners
• Running lubricant through the bearings of process machinery, turbines, reduction gears, and engines.

Before we jump into the general selection guidelines of rotary pumps, I think we need to understand several terms that related to rotary pumps, such as slip, volumetric efficiency, and mechanical efficiency, and of course several types of rotary pumps.

Slip

Many manufacturers rate rotary pumps by capacity. Capacity is displacement minus slip.

Then, what is slip?

Slip (S) is the fluid quantity that leaks from high-pressure discharge side to low-pressure suction side. Therefore, the actual capacity of a rotary pump is less than the calculated (theoretical) capacity. The theoretical capacity is reduced by recirculation back through the clearances between rotors and casing. We call the recirculation fluid as “slip”.

The delivered capacity (Q) of a rotary pump is calculated by the following equation:

Q = Q_t – S

Where:

Q = delivered capacity

Qt = theoretical capacity

S = slip

Slip occurs because all rotary pumps require clearances between rotating elements and the pump housing. The larger the clearances, the larger the slip is. Large clearances can be due to machining tolerances or wear.

Slip is directly proportional to the pressure differential across the pump and inversely proportional to the fluid viscosity.

The capacity of rotary pump is reduced due to decreasing viscosity, increasing differential pressure, increasing internal clearance between stationary parts and rotating parts, and decreasing pump speed.

Volumetric Efficiency

Volumetric efficiency is usually used by many manufacturers to express the amount of slip. The volumetric efficiency of rotary pumps is calculated by using the following equation:

E_v = Q / Q_t = (Q – S)/Q_t

Factors affecting volumetric efficiency of rotary pump are speed, viscosity, and differential pressure. Other factors include heating losses from fluid friction, mechanical losses in the bearings, and drag due to viscosity.

Mechanical Efficiency

Mechanical efficiency (E_m) of a rotary pump is:

E_m = HHP/HP x 100 = (Q) (DP) /1715

Where:

HP = input horsepower

HHP = hydraulic horsepower

Q = flow (gpm)

DP = differential pressure, psi

The mechanical efficiency of a rotary pump ranges from 60% to 70%, and can be as low as 50% under unfavourable condition, or as high as 80% under low slip and favourable conditions.

Rotary Pump Types

In general, there are three types of rotary pumps, which are eccentric rotor, gear, and screw. Each type has several variations.

Eccentric Rotor

Eccentric rotor pumps are divided into sliding vane type and flexible vane.

The sliding vane is the most used eccentric vane pump used in oil and gas industry. It consists of an eccentrically rotating drum with radial slots that house sliding vanes. The vanes slide in and out of the rotor. Sliding vane pumps work well with low-viscosity fluids and are somewhat self-compensating for wear. However, sliding vane pumps are inexpensive and unreliable due to frictional wear and vane breakage. They are not suitable for high viscous fluids.

The flexible vane is similar to the sliding vane except that the vanes are generally soft and are integral with the rotor. As the rotor turns, the vanes bend and conform to the eccentric shape of the cylinder. Flexible vane pumps can only be used in low-capacity, low-pressure, and non-critical service. They should not be allowed to run dry and should only be used with low temperature fluids.

Gear Pumps

Gear pumps are generally classified as external gear, internal gear, and lobe.

The external gear pump consists of two equal sized meshing gears, one driving and the other an idler, which rotates inside a housing. External gear pumps are compact in size, can produce high pressures, and well suited for highly viscous fluids. They are easily manufactured in a broad range of materials to ensure compatibility with pumped liquid. Due to the close tolerances (see about “Slip” above), external gear pumps should be used with clean fluids.

The internal gear pump has similar principle to the external gear except the drive shaft turns a ring gear with internal teeth. The internal gear pump is a reliable and inexpensive pump. The main disadvantage involves excessive backpressure or handling fluids that contains solids.

Lobe rotary pumps operate in the same principle as the gear pumps, except the rotating elements have two, three, or four lobes instead of gear teeth. Lobes never encounter each other, so the pump can be allowed to run dry. The large volume of voids created between the casing and lobes allow many products to be pumped without damaging the product itself. Lobe pumps have no metal-to-metal contact between the lobes, thus it is much less likely that wear-related traces of iron, steel, or other pump building components will wind up in the final output. They are more expensive than gear or vane pumps, challenging to repair and maintain, and expensive to use because timing gears are needed.

Screw Pumps

In the oil and gas sector, screw pumps are by far the most often utilized rotary pumps. Screw pumps are divided into single-screw, two-screw, and three-screw. Screw pumps can be single or multiple-rotor design.

The single-screw is a slow-speed pump and physically large for fluid being pumped. It can be used in viscous fluids and fluids with high solids content. They can handle fluids varying from clean water to sludge without changing clearances or components and offer uniform discharge pressure with low pulsation. Single-screw pumps are expensive, large, bulky, and hard to maintain, and require expensive replacement parts.

The two-screw pump has two screws that rotate within the stator or casing. The two screws are designed with greater clearance, sot they rarely contact the stator. The advantage of the external-bearing two-screw over the three-screw is that it is less susceptible to wear in services with suspended solids, but the trade-off is higher cost.

The three-screw pump has three screws that rotate within the stator or casing. In small sizes, they are used to supply lubricating oil to engines and machinery. In large sizes, they are used to load and unload barge and tankers.

General Selection Guidelines for Rotary Pump

Table below show general selection guidelines for rotary pump.

Pump Description	Two-screw with ext. three-screw bearing		Single-screw	External gear w/int. bearings	External gear w/ext. bearings	Internal gear
Differential pressure range (psi)	3000	1500	200	70	700	200
Flow range (gpm)	1000	5000	450	600	600	150
Max. temperature (F)	500	500	180	250	250	250
Horsepower range (HP)	800	2000	800	400	400	40
Speed range (RPM)	3600	1800	800	1800	1800	1800
Viscosity range (SSU)	1,000,000	1,000,000	1,000,000	250,000	250,000	250,000
Variable viscosity	No	Yes	Marginal	No	Marginal	No
Relative cost to purchase (note 1)	3	5	4	2	3	1
Relative cost to install (note 1)	3	4	2	2	3	1
Relative cost to maintain (note 1)	4	3	5	2	3	1
Self-priming	Yes	Yes	Marginal	Yes	Yes	Yes
Can run dry – short time	No	Marginal	No	No	Marginal	No
Field alignment required	Yes	Yes	Yes	Yes	Yes	Yes
Good for some entrained gas	Yes	Marginal	Yes	Yes	Yes	Marginal
Good for abrasives	No	No	Yes	No	No	Marginal

 Note 1: relative number (1-5, 1 is the best).

That’s all about general selection guidelines for rotary pumps. I hope you find this post useful as usual.

Reference:

Stewart, Maurice, Surface Production Operations- Pumps and Compressors Volume Four, Gulf Professional Publishing, 2019.