Centrifugal Pump

Written by Matt Milbury. Posted in Pump

A basic centrifugal pump is a very basic piece of equipment in the oil field. It consists of a motor, casing and an impeller. A centrifugal pump converts rotational energy into pressure energy. This is accomplished by accelerating the fluid across the vanes of the pump's impeller.

There are several types of centrifugal pumps.

ANSI Pumps

The most common is the ANSI pump (in the US) and an ISO pump (everywhere else). An ANSI Pump is such that there is a standard set of dimensional standards that describe the dimensions of a back pull out, centrifugal pump. The reasoning of an ANSI pump is to standardize the footprint of a pump so that one may be pulled out and another put in its place.

ANSI Pumps are typically limited to a lower flow rate and head than an API pump. For most low pressure applications, this is the pump to use. However, just like everything else, be sure to consult the client's specifications prior to calling for an ANSI Pump. The same holds true for API Pumps.

API Pumps

Like an ANSI Pump, an API Pump (governed by API Standard API 610) is built to a standard set forth by API. API Pumps are generally larger and more robust pumps than ANSI pumps. As an ANSI pumps is for use in the chemical industry where API pumps are for use in the oil & gas industry.

For installation help, click on the  Installation page.

For materials of construction, click on the  Material of Construction Page.

Advantages

Centrifugal pumps have many advantages over other types of pumps. Among them are:

  • Steady flow (No pulsation)
  • Supplies the head (pressure) required by the system
  • Excellent rangeability and controllability with a VFD or throttling valve
  • Can handle most liquids with some solids
  • Can be operated with totally blocked flow or shut off temporarily without damaging the pump.
  • Standard design means parts are usually readily available
  • Very reliable

Disadvantages

  • Limited to low to moderate viscosity fluids
  • Not to be used for oil water emulsion as this pump exerts shear forces on the fluid and may tighten the emulsion.
  • Developed head is limited.
  • Must be primed prior to start up (non self priming)
  • Pumps are sensitive to NPSH
  • Fluids with entrained gas may risk cavitation. (Always check vapor pressure when doing the NPSH calculation)

Centrifugal Pump Control

The use of a throttling valve in place of a PLC can often times be an inexpensive solution for controlling the discharge of a centrifugal pump. However, this is not always a good solution. While using a throttling valve, back pressure is held against the discharge of a centrifugal pump. The pump runs on its curve, regardless of the efficiency. This is much like running your car at full throttle and controlling your speed with the break pedal. Operation of a VFD is using the throttle to speed your vehicle up and down.

  • VFDs eliminate requirement for engineering, design, installation, operation and maintenance of control valves. Add the supporting pneumatics, tuning and servicing required (leaks).
  • When throughput rates are constantly changing, the process can throttle itself back, allowing the pumps to run at lower speeds. Note, even with throttling valves, there is a minimum flow rate that must go through a centrifugal pump. In some cases, flow control valves may result in reduced flow rates below pump manufacturer recommended minimums, potentially causing damage do to overheating, etc.
  • VFDs provide a single non-intrusive point for both flow and pressure control. This should be taken into consideration when the process is such that requires very expensive valves or requires the minimization of leak paths (such as acid service).
  • VFDs provide a significant reduction in hydraulic horsepower at lower flows and heads verses a using a throttling valve
  • VFDs are promoted by many utilities as a method of energy conservation and rebate programs may be available to offset the cost of the drive.
  • VFDs offer many other fringe benefits:
    • Multiple measured variables for dynamic motor monitoring/feedback
    • Higher degree of dynamic motor protection
    • Motor current (torque) control
    • Elimination of across the line starters
    • Employ less expensive fusing verses HMPCs required with starters
    • Increased motor and pump life 

Standards

  • ASME Standards
    • ASME B73.1 - Specification for Horizontal End Suction Centrifugal Pumps for Chemical Process
    • ASME B73.2 - Specifications for Vertical In-Line Centrifugal Pumps for Chemical Process
    • ASME B73.3 - Specification for Sealless Horizontal End Suction Metallic Centrifugal Pumps for Chemical Process