A brief presentation on “CENTRIFUGAL PUMP WITH SPEED CONTROL”

Discussion Points:

  • HOW ELECTRICAL MOTOR WORKS?
  • BASIC OF CENTRIFUGAL PUMP OPERATIONS
  • CONVETIONAL WAYS OF PROCESS CONTROL
  • SPEED CONTROL SYSTEM BASICS
  • HOW VFD/VSD SAVES PUMPING ENERGY

How Electrical Motor Works:

  • Rotor has fixed magnetic fields
  • Stator receives current from the drive which creates a magnetic field.
  • This rotating magnetic field moves the rotor
  • The frequency is how often the current flows through the stator.
  • Synchronous Speed = (120 * frequency) / # Poles =(120 * 60Hz ) / 4          = 1800 rpm
Schematic of Electric Motor
Schematic of Electric Motor

Methods of Pump speed control:

Variable Frequency drive (VFD)-

  • It converts fixed frequency three phase Power input to variable frequency three phase output to Motor.
  • Output frequency is greater or less than supply frequency.
  • Generally frequency is lower or same as supply frequency  due to motor constraints.
  • Motor Speed = (120 * frequency) / # Poles
Variable Speed Drive
Variable Speed Drive

Basis of Centrifugal Pump Operation:

  • Pump characteristics curve (H-Q curve)-Pump specific
  • System resistance curve- System specific
Pump H-Q Curve
Pump H-Q Curve

Conventional way of Flow control:

  • By installing Control valve at pump discharge and which will be throttled to controls the required flow.
  • Pump will follow the pump curve
Varying pump capacity
Varying pump capacity

Flow Control using speed control:

  • By changing the pump speed using VFD / VSD, Pump flow rate will change.
  • Pump will follow the system resistance curve.

Behaviour of pump speed control :

  • VFD/VSD follows the system curve by changing pump speed.
  • VFD/VSD pump retains its characteristic performance curve shape.
  • % change in speed changes same % of flow rate if no static head is available.
Effects of Changing Speed of a Centrifugal Pump
Effects of Changing Speed of a Centrifugal Pump
  • Pump can be operated within the operating envelop provided by the pump vendor, flow and corresponding head can be achieved within operating envelop along with system resistance curve.

Specifying Speed control pumps:

  • Like any fixed speed pump VS pumps also required proper sizing.
  • To achieve all desired  head Vs flow ranges for VS pump all anticipated  cases shall be provided to vendor for all Sp.Gr. (fig. shows a pump can not reach point B)

Pump sizing

Specifying VFD pumps:

  • NPSHr and Motor HP shall be adequate for all flow and head range.
  • NPSHr also depend on viscosity and its effects shall be confirmed with pump vendor.
  • At lower flow rate generally HPSHr increases, follows bathtub curve and same shall be confirmed with vendor.

Benefits of Speed control:

  • No need of throttling valve to operate the pump at desired flow and head.
  • Control By-pass valve / MCF valve can be avoided.
  • Reduced capital and maintenance cost of throttling valve and bypass valve.
  • Reduction in required head (Delta P required for throttling valve)
  • Power saving at reduced flow requirements
  • Reduced heat dissipated to pumping fluid for fixed speed pump as efficiency is lower at reduced flow.
  • Start-up of high HP machine i.e compressor, Pump, etc. at lower speed.

Why different operating points?

  • Over sizing of pump by adding safety margin.
  • Reduced production over the life cycle.
  • Different destinations- one at a time.
  • Changes in physical properties of fluids.(emulsions with different water cut)
  • Changes in piping roughness over a life.
  • Single pipeline used for multiple products.
  • Fluctuations in destination pressure (MOL line)

When VFD/VSD application is economical?

  • If static head is lower than 50 % of total head required. Required flow is 50% of design value and may required for 20% of time.
  • If reduced duty is between 60-80% of BEP (Proper sizing of pump for required duty) Only VFD application may not sufficient for control range

Additional requirement for Motor with VFD:

  • Motor operated on VFD generates higher Temp. due to irregular shape of electrical waveforms produced by VFD. Thus motor efficiency will reduced due to high operating temperature
  • To ensure motor will not overheat, motor are typically de-rated at full load from 3 to 10%.
  • Increase heat can lead to environmental hazards as motor skin temperature may increase. Specially design motor or de-rated motor shall be used.
  • Some time special motor bearing shall be proposed to work in high temperature environment.
  • Generally motor shall be specified as suitable for VFD and shall purchased from pump supplier.
  • May need additional cooling fan for motor cooling at low motor speed.
  • Additional Capex for VFD
  • Need additional space in control room for VFD installation
  • Additional Air conditioning requirement .
  • If installed outdoor cost of additional enclosure and additional air conditioning is very high.

VSD vs VFD?

  • VSD is economical up to 80% of speed. losses increases below 80% speed and VFD becomes only economical option compare to VSD.
  • VFD can be configured with a fail-safe characteristics and pump can run at full speed, however VSD can not .
  • VFD can be by-passed if full speed operation is required, however the VSD is dedicated to machine and can not be bypassed.

Is it worth?

  • Despite the need of additional Capx and special requirement, VFD/VSD pumping can save money. But System economics shall be studied properly.
  • VFD/ VSD eliminates requirement of throttling valve, Bypass valve, Motor starter and reduces power requirement shall be properly consider while analyzing economics.
  • For Start-up of high HP machines – VFD/VSD eliminates requirement of additional power turbine in some cases.

Typical capacity control:

  • Suction pressure control as a capacity control
  • Discharge pressure control as a high pressure control.
  • Station capacity is controlled by pressure or level which manipulates the pump speed through cascade control to maintain desired suction pressure / tank level.

End of curve control:

  • End of curve operation could be a concern in pumping in large oil pipeline where the back pressure (system resistance) can very.
  • For fixed speed pump, based on flow the pump minimum and end of curve operation can easily be defined.
  • For variable speed pump, the flow value is not an indication of pump operating point in the performance curve. Hence pump parameter is defined based on flow and head.
  • Pump parameter X is defined as X= K* (flow)^2/Delta P
  • Pump Parameter is a constant line on performance map which defines the pump relative operating point
  • If system resistance curve is very steep, it is not possible to control / maintain the pump end of curve operation using speed.
  • Once system resistance move away from performance map, control system needs another handle to increase the resistance.
  • This is generally achieved by control valve on common discharge line.
Pump Performance Curve
Pump Performance Curve
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Anup Kumar Dey

I am a Mechanical Engineer turned into a Piping Engineer. Currently, I work in a reputed MNC as a Senior Piping Stress Engineer. I am very much passionate about blogging and always tried to do unique things. This website is my first venture into the world of blogging with the aim of connecting with other piping engineers around the world.

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