Piping Basics - Introduction to pumps (Part 1)

Pumps are used in production facilities to move liquid from a low pressure or low elevation location to one of a higher pressure or elevation. Wherever possible it is usually advantageous to locate equipment and select operating pressures in such a manner as to minimize the need for pumping or to minimize the volume for pumping.

Pump classification

Kinetic pumps are referred as constant pressure pumps as the discharge pressure through these pumps cannot be varied without changing the physical conditions of the pump (such as throttling of discharge valve or increasing the size of impeller).

Positive displacement pumps are referred as constant discharge pumps because the volume/gap between the piston & cylinder in these pumps is constant.

One can increase the pressure by closing the discharge valve and increasing the stroke length of the piston. This increase in stroke length will continuously increase the discharge pressure.

1-  Kinetic pumps

In a kinetic pump, energy is added continuously to increase the fluid's velocity within the pump to values in excess of those that exist in the discharge pipe. Passageways in the pump then reduce the velocity until it matches that in the discharge pipe. From Bernoulli's law, as the velocity head of the fluid is reduced, the pressure head must increase. Therefore, in a kinetic pump the kinetic or velocity energy of the fluid is first increased and then converted to potential or pressure energy. Almost all kinetic pumps used in production facilities are centrifugal pumps in which the kinetic energy is imparted to the fluid by a rotating impeller generating centrifugal force.

1.1 Centrifugal pumps

Centrifugal pumps are classified as either radial flow, axial flow or mixed flow. In radial flow pumps, the flow enters the center of the rotating wheel (impeller) and is propelled radially to the outside by centrifugal force. Within the impeller the velocity of the liquid is increased, and this is converted to pressure by the volute case.

In a typical axial flow pump, flow is parallel to the axis of the shaft. A velocity is imparted by the impeller vanes, which are shaped like airfoils.

Most pumps are neither radial flow nor completely axial flow but have a flow path somewhere in between the two extremes. Radial flow pumps develop a higher head per stage and operate at slower speeds than axial flow pumps. Therefore, axial flow designs are used in very high flow rate, very low head applications.

By varying the pump speed, the throughput at a given head or the head for a given throughput can be changed. In Figure below as the speed decreases from N1 to N2 to N3, the flow rate decreases if the head required is constant, or the head decreases if the flow rate is constant.

For the system shown in the system-pump interaction curve, as the pump is speeded up or slowed down a new equilibrium of head and flow rate is established by the intersection of the system curve with the pump curve.

The throughput can be changed by imposing an artificial backpressure on the pump, by using a control valve.

By adjusting the orifice, the control valve can shift the system curve, establishing new head-flow-rate equilibria. As the pressure drop across the control valve increases, the flow rate through the system decreases.

The advantages of centrifugal pumps are:

1. They are relatively inexpensive.

2. They have few moving parts and therefore tend to have greater onstream availability and lower maintenance costs than positive displacement pumps.

3. They have relatively small space and weight requirements in relation to throughput.

4. There are no close clearances in the fluid stream and therefore they can handle liquids containing dirt, abrasives, large solids, etc.

5. Because there is very little pressure drop and no small clearances between the suction flange and the impeller, they can operate at low suction pressures.

6. Due to the shape of the head-capacity curve, centrifugal pumps automatically adjust to changes in head. Thus, capacity can be controlled over a wide range at constant speed.

Although several impellers can be installed in series to create large heads, centrifugal pumps are only practical for achieving high pressure when there are large flow rates. In addition, centrifugal pumps have low maximum efficiencies when compared to reciprocating pumps. Since the efficiency also declines as the flow rate varies from the design point, in actual operation, the pump will operate at still lower efficiencies. Efficiencies between 55% and 75% are common.

2-  Positive displacement pumps

Volume containing the liquid is decreased until the resulting liquid pressure is equal to the pressure in the discharge system. That is, the liquid is compressed mechanically, causing a direct rise in potential energy. Most positive displacement pumps are reciprocating pumps where the displacement is accomplished by linear motion of a piston in a cylinder. Rotary pumps are another common type of positive displacement pump, where the displacement is caused by circular motion.

2.1 RECIPROCATING PUMPS

In reciprocating pumps, energy is added to the fluid intermittently by moving one or more boundaries linearly with a piston, plunger, or diaphragm in one or more fluid-containing volumes. If liquid is pumped during linear movement in one direction only then the pump is classified "single acting." If the liquid is pumped during movement in both directions it is classified as "double acting."

The difference between a piston and plunger pumps is the high-pressure seal. In a piston pump, the seal is attached to and reciprocates along with the piston. Because of this, piston pump seals wear out faster and cannot handle as much pressure compared to plunger pumps which are usually used in municipal and industrial sewage.

The head-flow-rate curve is a nearly straight vertical line. That is, no matter how high a head is required, the plunger will displace a given volume of liquid for each rotation. The flow rate through the pump can only be varied by changing the pump speed. A throttling valve that changes the system head-flow-rate curve will have no effect on the flow rate through the pump. Since the pump will attempt to match whatever system pressure is required, it is necessary that a relief valve be installed on the pump discharge. This assures that the pump will not overpressure itself or the discharge pipe.

The advantages of reciprocating pumps are:

1. The efficiency is high regardless of changes in required head. Efficiencies on the order of 85% to 95% are common.

2. The efficiency remains high regardless of pump speed, although it tends to decrease slightly with increasing speed.

3. Reciprocating pumps run at much lower operating speeds than centrifugal pumps and thus are better suited for handling viscous fluids.

4. For a given speed the flow rate is constant regardless of head. The pump is limited only by the power of the prime mover and the strength of the pump parts.

Because of the nature of their construction, reciprocating pumps have the following disadvantages when compared to centrifugal pumps:

 1. They have higher maintenance cost and lower availability because of the pulsating flow and large number of moving parts.

2. They are poorer at handling liquids containing solids that tend to erode valves and seats.

3. Because of the pulsating flow and pressure drop through the valves they require larger suction pressures (net positive suction head) at the suction flange to avoid cavitation.

4. They are heavier in weight and require more space. 5. Pulsating flow requires special attention to suction and discharge piping design to avoid both acoustical and mechanical vibrations.

In reciprocating pumps, the oscillating motion of the plungers creates disturbances (pulsations) that travel at the speed of sound from the pump cylinder to the piping system. These pulsations cause the pressure level of the system to fluctuate with respect to time. In order to lessen potentially damaging pulsations in piping from this pressure fluctuation, pulsation dampeners may be installed in the suction and/or discharge piping of the reciprocating pump.

2.2 Diaphragm pumps

Flexure of the diaphragm creates the pumping action. When gas pressure is applied against either diaphragm it forces liquid out. When the gas is relieved the diaphragm flexes under the pressure in the suction line and allows liquid to enter. The advantages of a diaphragm pump are that it can handle large amounts of suspended solids, is inexpensive to repair, can handle low flow rates inexpensively, and can run periodically without any liquid. However, diaphragm pumps require frequent maintenance because they are reciprocating pumps and because the diaphragm has a tendency to fatigue with time. They generally cannot handle very high flow rates, or discharge pressures.

2.3 Rotary pumps

These pumps operate by having a rotating member turn inside a housing in such a way as to create trapped liquid through the pump. Although these pumps may look like centrifugal pumps, their action is that of a positive displacement pump in that the liquid is continually compressed to a high pressure without first being given a high kinetic energy.

Rotary pumps have the same characteristics as reciprocating pumps, except that at low speed, leakage between the cavities increases. At very low speeds the reduction in efficiency can be very significant. When compared to reciprocating pumps, rotary pumps require less space, and deliver relatively pulsation-free flow. Their main advantage is that unlike reciprocating and centrifugal pumps, their construction subjects the pumped fluid to a minimum amount of shear or turbulence. Thus, they tend to be used in process applications where one of the other pump types could be expected to shear and disperse one liquid into another making subsequent treating more difficult. Their disadvantages are that they have close clearances that require that the liquids being pumped have a lubricating value, be non-corrosive, and contain few solids. Therefore, they tend to be limited to relatively solids-free oil or emulsion streams.

Multiphase pumps

Multiphase fluids typically produced from an oil well consist of hydrocarbon liquid, hydrocarbon gas, and an immiscible water phase. These fluids historically must be processed by a multiphase production system near the wells. This arrangement is needed because transfer of the multiphase fluids is achieved through the use of reservoir energy and, in most cases, this energy is insufficient to transfer fluids over any considerable distance. The inherent problem with processing multiphase fluids close to the wells is the high capital and operating cost experienced (both onshore and offshore). A solution to this problem is to obtain a pump that can handle unprocessed multiphase fluids and transport them over considerable distances; the multiphase pump can do this task. This pump is able to boost the pressure of wellhead fluids over considerable distances to a central processing facility, thereby eliminating several smaller local processing facilities. In addition to providing for economic savings from consolidation of surface and offshore facilities, the use of multiphase pumps makes the development of satellite fields more economically attractive. Multiphase pumps also aid in increasing well production rates by lowering required back-pressure on wells. Multiphase pumps are applicable in services where the gas volume fraction (GVF) is as high as 95%. To determine the GVF, divide the actual gas flow rate by the total mixed flow rate. For a GVF above 95%, the volumetric efficiency decreases, and more fluids (gas) slip back to the pump inlet. Slower speed and higher pressure boost also increase slip, and thus decrease volumetric efficiency.