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Plenty Mirrlees Pumps: 2000 Series - CPC PumpsEnergy conservation is of concern and has become increasingly important in all aspects of equipment design, application and operation. Thus innovative energy conserving approaches should be aggressively pursued... evaluation of purchase options will increasingly be based on the total life costs versus low initial cost. This statement is contained in the foreword to API 676, an industry standard for positive displacement pumps.
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An area where potentially High Energy savings can be made is in the application of variable flow rate 2000 Series Pumps with Constant Pressure Control (CPC) to systems and processes where the fluid demand is not constant. Examples of this are:
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This diagram shows a fixed capacity pump installed in a system where the demand at B is either variable or intermittent. As the flow rate from the pumps is fixed, the varying demand at B requires the excess to be recirculated. This is generally achieved using a pressure control valve (relief valve) to return the excess to the suction side of the pump. Power input to the pump is proportional to flow and pressure, therefore, as the flow from the pump is always fixed and the pressure will rise marginally when reciprocating (due to valve accumulation), the power input will be higher than the normal running power, i.e. if 30% of the flow is recirculated then approximately 30% of the power input is wasted and dissipated as heat. |
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This diagram hows a variable flow rate 2000 Series Pump with Constant Pressure Control (CPC) in a similar system. In the variable flow 2000 Series Pump, the variation is achieved by altering the eccentricity of the rotor relative to the centreline of the shaft. This therefore modifies the basic theoretical throughput and does not require any shaft speed variation. Because the theoretical flow rate is varied, heat generation at low flow rates is minimal. |
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The system would therefore operate as follows. With the pump operating normally, the full flow is delivered at B. If demand at B is reduced, the pressure at B would rise and the pump, sensing this, would reduce its eccentricity almost instantaneously, thus reducing harmful pressure peaks.
It is the fast response and simple construction of the Constant Pressure Control 2000 Series Pump that enables the energy saving features to be best exploited. The Constant Pressure Control operates by hydraulically sensing the discharge pressure and when it rises above a preset (and adjustable) reference value the eccentricity and therefore flow is reduced until flow rate matches demand and pressure is constant. With zero demand the pump is in the neutral position and delivers no flow. The fast response of the Constant Pressure Control ensures that system pressure peaks do not occur. This is of particular importance in drum/can filling applications where the change from full to zero demand occurs rapidly.
Considering the complete systems shown and assuming the pump is running and producing a flow rate and gas pressure is equal to discharge pressure, it can be seen that if the valve is closed against the pump, the delivery pressure will increase causing the piston to move up lifting the inner bodies reducing the capacity which in turn means the delivery pressure will reduce to become equal to the gas pressure again. Therefore the pump capacity is automatically reduced although the delivery pressure is maintained.
If the valve is completely closed the piston is arranged to stop when the 'no flow' position is reached. Conversely if the valve is opened the delivery pressure will fall causing the piston to move down increasing the pump capacity and consequently the delivery pressure will increase to become equal to the gas pressure again.
Rolling Diaphragm Concept The rolling diaphragm is a tough flexible seal with a unique configuration that permits relatively long piston strokes while completely eliminating sliding friction. Formed in the shape of a truncated cone or top hat, the diaphragm is turned in on itself when installed so that during the stoke it rolls and unrolls alternatively on piston skirt and cylinder wall. The rolling action is smooth and effortless, completely eliminating sliding contact and breakaway friction. With the outer flange clamped to the cylinder and the centre fastened to the piston head, the rolling diaphragm forms a perfect barrier, preventing blow-by leakage and pressure loss. Unlike 'O' rings, cup seals and other conventional sealing devices it requires no lubrication of any kind. Depending on the materials selected, rolling diaphragms can function effectively with operating temperatures between –84°C and +315°C, and are highly resistant to oil, ozone, acids, alkalis, steam and other corrosive fluids. Rolling Diaphragm Construction Rolling diaphragm material is essentially a layer of specially woven fabric, impregnated with a thin layer of elastomer. The total thickness is usually 0.5 to 1mm. The fabric which lends high tensile strength to the diaphragm is designed to permit free circumferential elongation (allowing free rolling action while preventing axial distortion). This eliminates stretching or ballooning during the stroke. The flexible life of the rolling diaphragm depends on operating pressure, amount of axial and circumferential stress applied during the stroke, and the materials of construction. In general, a properly installed diaphragm will provide a life of many millions of cycles.
In general, variable flow rate pumping may save costs in a number of ways. For example, motors are often oversized to cope with the infrequent requirements of cold starts. The high viscosities associated with these will increase pipeline frictional losses and therefore the pump discharge pressure. This increases the required driver power size by an amount in proportion to the pressure rise. If flow rate is not critical it may be reduced at start up to reduce the absorbed power to an acceptable level. Likewise if a factories power costs are related to various tariffs dependent on peak demand, the flow of the pump may be adjusted to bring the power close to the maximum for that tariff band without jumping into the next band. The Constant Pressure Control system also offers cost savings in other ways. The cost of the pressure control valve and associated pipework is removed and the floor space these take up is saved. The variable flow pumps fitted with either hand or remote control are also particularly suited to other systems. Bitumen and grease production, for example, require the flow away from the production process to be varied in accordance with the rate of production. Blending systems are another application where a variable flow rate is important and can simplify the quality control of the finished product. An additional benefit here is the low shear characteristics of the 2000 Series Pump which reduces foaming of the liquid. The reverse flow features of some variable flow pumps offer benefits in some systems. For example, road/rail cars may be loaded or unloaded using the same systems. Also by using reverse flow, lines handling a variety of products may be stripped to reduce cross contamination. There are, of course, other ways of achieving variable flow rate pumping. One common way is to drive a fixed flow pump with a variable speed drive. The limitations here are infinite variation right down to zero is difficult to achieve, response time can be slow and initial cost can be very high. Also, as most contain sophisticated electronics, the skill of maintenance and supervisory personnel has to be higher. Large and small companies worldwide are using variable flow rate 2000 Series Pumps to their advantage. Flow rates up to 500m3/hr (from a single pump) and discharge pressures up to 40 bar are possible. Flow can also be controlled manually with a handwheel or remotely using an electric or pneumatic actuator. A wide range of liquids from many industries can be handled. Suction capabilities are also excellent. |
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If demand is zero then the power input is only the
small value required to overcome friction in the pump and drive
plus the small power absorbed by internal slippage. Compare
this to the fixed flow rate pump system where zero demand requires
power input of approximately 10% to 20% higher than the normal
running power, i.e. 110% to 120% of the normal running power
is being wasted. Significant power savings are therefore possible
in systems where demand is not constant if a Plenty 2000 Series
CPC Pump is used.
The inner bodies are attached to
a piston via the bridge block. Pumped liquid at discharge pressure
is fed to the underside of the piston and sealed by a rolling
diaphragm. The piston is housed in a cylinder which is charged
with an inert gas and sealed by another rolling diaphragm.
