Many times, you may be faced with the decision to use a reciprocating pump or a multi-stage centrifugal pump for a given application. There are some pros and cons for either pump design, depending on:
- Where the pump is to be installed
- The fluid being transferred
- Future expansion plans
- Variable system parameters
- Operation and maintenance issues
- Personnel knowledge and experience with the pumps
1. Operating Principles
A centrifugal pump adds kinetic energy to a fluid by means of fast-rotating impellers. There is no fixed volume, and the fluid increases in kinetic energy (velocity) while moving through impeller passages by centrifugal force resulting from impeller rotation.
Its accelerated velocity is converted into pressure head by exiting into the diffuser for discharge, or – in the case of a multi-stage centrifugal pump – it further increases its velocity (pressure head) by moving through to the next fast-rotating impeller (see Figure 1). Centrifugal pumps are usually direct coupled with drivers (electric motors or engines) without speed reduction.
A reciprocating pump utilizes a crankshaft-connecting rod mechanism identical to internal combustion engines. The crankshaft-connecting rod mechanism converts the rotary movement of the crankshaft to a reciprocating linear movement of plungers or pistons. The plunger/piston movement creates volume changes.
As a cavity opens when a plunger/piston retracts, the fluid is admitted through an inlet check valve. When the plunger/piston reverses, the inlet check valve closes, and the cavity reduces when the plunger/piston extends. The outlet check valve opens and the fluid is forced out by the plunger/piston.
The discharge volume is fixed for each crankshaft revolution, regardless of the fluid being pumped. Pressure is determined by the system flow resistance and pump construction (see Figure 2). Speed reduction is needed for decreasing high speed from the driver to low pump shaft speed.
2. Capacity and Head (Pressure)
Since a centrifugal pump has no fixed volume (as indicated above) at a fixed inlet size and casing, increasing the diameter of the impeller or the rotational speed will lead to increased head and increased flow rate.
A centrifugal pump can have a large capacity with a small footprint compared with a reciprocating pump. Of course, increased capacity will consume more energy. Capacity is proportional to impeller speed and diameter. Larger impeller diameter (higher exit velocity) (see Figure 3) and/or faster rotation speed will increase the head due to conversion of velocity to head.
Figure 3. A typical performance curve of a centrifugal pump.
Head is proportional to the square of the impeller diameter or speed. Further increase of head could also result from more stages of impellers. More stages also mean more friction loss between inlet and outlet. This friction loss is what usually limits multi-stage centrifugal pumps to low pressure in comparison with reciprocating pumps. Multistage centrifugals are normally preferred in high volume, low pressure applications.
Reciprocating pumps produce a fixed discharge volume for each crankshaft rotation, regardless of the fluid being pumped. Increased capacity can be achieved by increasing the plunger/piston diameters or increasing the rotational speed of the crankshaft (energy consumption increases as well).
However, there is limitation because of maximum allowed speed (usually low speed) and the space available with the pump power frame design. As a result, reciprocating pumps have a large foot print with equivalent capacity compared with centrifugal pumps due to its fixed discharge volume.
Reciprocating pumps can be rated at much higher pressures than multi-stage centrifugal pumps. Often a 40,000-psi plunger pump is utilized for higher pressure industrial water blasting. A 10,000-psi to 20,000-psi plunger pump is often seen at oilfields for well service applications. Also, 800-psi to 5000-psi applications for other industrial and oilfield services are common. A typical reciprocating pump performance chart is shown in Figure 4C.
Reciprocating pumps do not generate pressure. The pressure seen at the pump is the result of resistance downstream of the pump discharge. Pressure can be as high as possible, as long as the plunger load is within rated values and the pressure is within the fluid cylinder and piping rated pressures.
In a reciprocating pump, small plungers are rated for higher pressures for a given rated plunger load because the area subjected to the pressure load, the end cross area, is smaller. Discharge pressure is rated for the complete speed range. Usually, speed changes do not affect the pressure rating for a continuous duty application. Reciprocating pumps are normally preferred for lower volume, high pressure applications.
As discussed above, centrifugal pumps transfer fluid by high speed rotating impellers to convert kinetic energy into pressure energy. There are no direct energy transfers. Basically, there are three types of efficiencies to consider: Volumetric, Hydraulic, and Mechanical.
Volumetric efficiency loss is due to leakage in the impeller-pump casing clearance.