Oil Gas Plant Centrifugal Pump Operation

Centrifugal Force

Suppose a bucket that is almost completely filled with water is rotated in a circular motion around a body.  If it rotates very slowly, some of the water will spill out of the bucket.  However, if it rotates fast enough, none of the water will spill out of the bucket.  The centrifugal force generated by rotating the bucket pushes the water against the bottom of the bucket so that is does not spill out.

Now suppose there is a small hole in the bottom of the bucket.  As the bucket rotates, water will come out of the hole.  The faster the bucket rotates, the farther the water will travel when it leaves the bucket.

FIGURE 15

CENTRIFUGAL ACTION

This is the principle of centrifugal action.  When the bucket moves fast, more energy is used.  the distance that the water travels from the hole in the bucket will depend upon the amount of energy that is used in rotating the bucket.

A typical centrifugal pump unit includes a driver and a pump.  The energy used by the driver motor, turbine, or engine is transferred to liquid in the pump in the form of pressure by the pump.  In other words, a pump is a device for transferring energy from the driver to the liquid.  It is important to recognize that energy is entering the liquid in order to understand pumping.

Electrical energy used by a motor-driven pump is transferred to liquid by the pump in the form of pressure.

Another point to realize is that energy can exist in several forms.  A rifle shell contains energy in the form of powder.  When the shell is fired, energy of the burning powder transfers to the bullet in the form of velocity.  That energy converts to pressure when the bullet strikes an object and losses its velocity.  Velocity energy is converted to pressure energy.

A centrifugal pump uses the same velocity-pressure concept to increase liquid pressure.  Liquid enters an impeller at the eye.  The speed of the impeller creates a centrifugal force that throws the liquid to the outer edge at a high velocity.  Liquid leaves the impeller at high velocity and enters the volute, which is an enlarged chamber where the velocity is quickly reduced.  This velocity reduction results in a pressure increase.  Refer to Figure 16.

The liquid flow can be compared to that of the moving bullet.  The flow in the impeller at a high velocity corresponds to the movement of a bullet through the air.  The liquid slowing down in the volute with a resultant pressure rise is comparable to the force of a bullet striking an object.

FIGURE 16

CENTRIFUGAL ACTION

The amount of pressure an impeller will develop depends upon its diameter and the speed at which it rotates.  A large diameter impeller operating at a high speed will develop the highest pressure.  The pressure developed by the impeller is limited by the materials of which the impeller is made.  It is subject to the same centrifugal force as the liquid and will fly apart if the centrifugal force is excessive.

If a single impeller will not develop the pressure required, two or more impellers can be installed in series to increase the pressure rise across the pump.  A pump with three impellers can be compared with three pumps which operate in series.  Discharge liquid from the first pump enters the second one, and liquid from the second pump flows to the third one.

There is no theoretical limit to the number of impellers which can be installed in a pump.  However, horizontal pumps seldom have more than eight impellers in one casing.  If this is not enough to produce the desired pressure, a second pump will be used.  Submersible or can pumps can have 50 or more impellers.  Vertical pumps are usually built in segments, so that there is no theoretical mechanical limit to the number of impellers which can be installed.

3.2     Liquid Head Pressure

Liquid Head Pressure is the pressure exerted by a liquid at the base of a column of liquid.  The pressure exerted by the column of liquid depends upon the height, or head and the relative density of the liquid.  Liquid head may be expressed in terms of pressure, and pressure may be expressed in terms of liquid head.

The purpose of a centrifugal pump is to increase the pressure of the liquid being pumped.  The head against which the pump is working is the Total Head which is made up from the Discharge Head and Suction Head.  Refer to Figure 16.

FIGURE 17

SUCTION, DISCHARGE & TOTAL HEAD

 Discharge Head

Discharge Head is the pump discharge pressure measured at the pump discharge and can be measured in psig, or head of liquid pumped.  Discharge Head comprises of three parts:

  • Static Discharge Head
  • Gauge pressure on surface of liquid in discharge vessel
  • Minus pressure lost in delivery pipe system.

Suction Head

Suction Head is the suction pressure measured at the pump suction and can be measured in psig, or head of liquid pumped.  Suction Head comprises of three parts:

  • Static Suction Head
  • Gauge pressure on the surface of the liquid in the suction vessel
  • Minus the pressure lost in the suction pipe system.

Total Head

Total Head is the actual increase in head that the pump is required to give to the liquid.

Therefore:-   Total Head   =   Discharge Head   –  Suction Head

A pump that develops a Total Head pressure of 45 psig, will have a discharge pressure 45 psig more than the suction pressure regardless of the suction pressure.

Suction Lift

Figure 18 shows a system where the liquid level in the suction is below the pump.  This is referred to a Suction Lift.

FIGURE 18

SUCTION LIFT HEAD

 3.3     Thrust

As a pump impeller rotates, a thrust force develops which is transmitted through the pump shaft.  See Figure 19.  The force developed in a single impeller pump is relatively low, and can be overcome with thrust bearings located on the pump shaft as shown in Figure 20A.

Single Impeller Exerts

Thrust Toward Suction End

FIGURE 19

EFFECTS OF THRUST

FIGURE 20A

THRUST

Thrust forces in multistage pumps are compounded at each impeller.  Special design considerations are required to reduce these forces.  One way of neutralizing two forces is to install some of the impellers in opposite direction to others, so the thrust forces equalize one another.  This design does not totally balance thrust forces, but it reduces them enough so that smaller thrust bearings can be used.  Figure 20B.

FIGURE 20B

EFFECTS OF THRUST

Some multi-stage pumps have all the impellers facing the same direction. These require special equipment to overcome thrust forces.

Pump discharge pressure acts on one face of the piston and pump suction pressure acts on one face of the piston and pump suction pressure acts on the other face of the piston, via a connecting balance line.  A small amount of liquid from the discharge leaks around the piston to the outer face, and flows to the pump suction.  The pressure on the outer face of the piston is almost the same as the suction pressure.  The force exerted on the inner side of the piston will equal discharge pressure times the area of the piston.  The piston is sized so that the net force resulting from the piston is approximately equal to, and in the opposite direction of, thrust force from the impellers.  This arrangement minimizes the size of thrust bearings required.  Figure 20C.

FIGURE 20C

THRUST

The balance piston is attached to the pump shaft and rotates in the casing.  The clearance between the piston and the casing must be very small to prevent excessive discharge liquid from leaking around the piston.  This requires a clean liquid inside the pump so that dirt does not get between the balance piston and the casing and wear one of the other parts.

Multi-stage pumps having opposed impellers require special passageways through the casing for liquid to flow from the final stage of the first set of impellers to the first stage of the opposing set of impellers.  This adds considerable cost to the casing.

Selection of a multi-stage pump having opposed impellers, or having in-line impellers with a balance piston, depends upon the pump service and the cost of the two arrangements.

Oil Gas Plant Centrifugal Pump Operation
Oil Gas Plant Centrifugal Pump Operation

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