Marine Auxiliary Machinery Chapter 3 Lesson 5 Centrifugal Pump operations
Learning Objectives • After successfully completing this lesson, you will be familiar with: • Pump and system curve • Series pumping • Parallel pumping • Test
Series Pumping - Multistage • Series pumping can be considered as a series of centrifugal pumps arranged to supply one another in series and thus progressively increase the discharge pressure. • The illustration shows a cross-section through a typical vertical multistage single entry centrifugal pump used deep-well cargo pumping.
Series pumping - operation • In series operation, the discharge of one pump feeds the suction of a second pump. When two or more pumps are operated in series, the flow through all of the pumps is equal. Since whatever flows through one pump has to flow through the next pump in series, provided there are no side streams.
Purpose for pumping in series (1) • Commercial equipment • One purpose for operating pumps in series is to insure that commercially available equipment can be used in a particular system, while at the same time reducing system costs.
Purpose for pumping in series (2) • Long pipelines • The system with a long pipeline and a large amount of friction loss is across the entire pipeline. • This would result in a pump with an extremely high head, and thus an extremely high horsepower.
Purpose for pumping in series (3) • NPSHA for the second pump • In this case, the first pump of two in series might be a fairly low head pump (and thus a fairly low horsepower pump). • But, in calculating NPSHA for the second pump in series, would have an additional term, namely the TDH (Total discharge head) of the first pump.
Series Pumping – Pump Curves (2) • Each of the pumps has H-Q curve • A combined pump curve can be generated. • This color curve is the combined curve for two pumps. • At arbitrarily selected values of flow, the TDH at this flow is doubled. • This color curve is for three identical pumps in series. • System head curves • The intersection of PC with SHC determines the total flow.
Non identical pumps in Series • The combined curve is generated using the same procedure as for identically sized pumps. • It is common in pipelines to have several differently sized pumps, allowing the operators the widest possible range of flow and/or variation of products pumped.
Pressure relief system • If a valve in the line downstream of the last pump in a series installation is inadvertently closed completely. • All of the pumps in the line move to their shutoff head. • The pressure in the system is considerably higher than normal • Possibly higher than the design pressure • A pressure relief system should be incorporated into the system
Parallel pumping (1) • The primary purpose of operating pumps in parallel is to allow a wider range of flow than would be possible with a single fixed speed pump for systems with widely varying flow demand. • Examples of applications for parallel pumping include: • Municipal water supply • Wastewater pumps • Pumps in water chilled heating, ventilating and air conditioning systems (HVAC) • Main process pumps in variable capacity process plant • Condensate pumps in a steam power plant
Parallel pumping (2) • Usually there are no more than three or four pumps operating in parallel. • When parallel pumps are being considered • The pumps must be carefully matched to each other • To insure that the pumps are always operating at a healthy point on their H-Q curves • To insure that the system is such that true benefits are achieved form the parallel pumping arrangement • However this does not always turn out be to the case.
Parallel pumping (3) • A combined pump curve must be developed depicting the head-flow relationship for the pumps while pumping in parallel. • Once these two curves are constructed, the rule for total system flow is that the total flow through the system is represented by the intersection of the system head curve with the combined pump curve.
Flow for two pumps in parallel (1) • The resultant flow when two pumps are operated in parallel in system is not double the flow which one pump alone produces when operating by itself, a sometimes mistaken impression.
Flow for two pumps in parallel (2) • If the system curve were completely flat, then the flow double. • But, because the system head curve curves up due to friction, two pumps in parallel don’t deliver double the flow of a single pump alone in the system. • Putting the third pump on line even further diminishes the increment of flow.
1pump in a system with 3 pumps (1) • With two identical pumps operating • The point ab is obtained by moving left from point b. • Point ab represents where each pump is operating on its own H-Q curve when two pumps are operated in parallel.
1pump in a system with 3 pumps (2) • Similarly, the dashed line moving left from point c intersects the pump curve at the point ac, where it would operate when three pumps operate in the system. • In general with parallel pumping, each pump runs out the furthest on its own H-Q curve when that pump operates alone in the system. • The pumps run the furthest back on their own H-Q curves when the maximum numbers of pumps are operated in parallel in the system.
Parallel – Non-identical Pumps • The curves of the two different pumps are combined in curve C • The combined curve C actually follows curve A for a while. • Point c determines the total flow of the two pumps. • The horizontal dashed line going left from point c intersects the individual H-Q curves at the point a and b respectively.
Parallel – System Mismatch (1) • Two non-identical pumps in parallel • The system curve Y is steeper • The pump A would operate at point a • The pump B would never be able to develop enough head. • It would cause the pump B • To be running at full speed but delivering no flow • Or caused pump B to operate at a very low flow.
Parallel – System Mismatch (2) • This type of mismatch of non-identical pumps in a system should obviously be avoided.
Steep system head curve (1) • When the system head curve is very steep, operating a second pump in parallel with the first produces only a marginal increase in flow. • If these same pumps were piped in series, rather than in parallel, they would produce a higher flow through the system. • Only two such pumps piped in series deliver more flow through the system than three pumps operating in parallel do.
Steep system head curve (2) • The general rule is: • If the system head curve is relatively steep, series pumping is probably more effective than parallel pumping for increasing the flow range of the pumps. • If fairly flat system head curves are concerned; parallel pumping is probably more effective producing wide flow range than series.
Steep system head curve (3) • If the system head curve is too steep, a situation which could be caused by an under-sized piping system or by some other undersized component in the system which acts as a bottleneck. • In this case, capital would have been better spent, if possible, in flattening the system head curve, reducing bottlenecks, and increasing piping sizes, rather than in adding additional parallel operating pumps to the system.
Question 4 • The main reason for operating pumps in parallel? • Is that a greater discharge pressure can be achieved. • Is to allow a wider range of flow than for a single arrangement • Is that the combined electrical load on the pumps is lower than an equivalent single pump.
Question 5 • Which one of the following statements is true? • Centrifugal pumps cover the lower ranges of specific speeds, axial flow the higher, and mixed flow the intermediate values • Centrifugal pumps cover the higher ranges of specific speeds, axial flow the intermediate, and mixed flow the lower values. • Centrifugal pumps cover the intermediate ranges of specific speed, axial flow the higher, and mixed flow the lower values.
Question 6 • Under what system conditions does it turn out that operating pumps in parallel achieves very little benefit? • If the piping system is of different size for each pump. • If the piping system is oversized, causing a lack of discharge pressure. • If the piping system is undersized or some component in the system acts as a bottleneck
Question 7 • The main reason for operating pumps in series is? • To avoid cavitation in high-pressure system. • To share the power absorbed using many small motors instead of one large motor, which is more energy efficient. • So that commercially available pumps (i.e. small) can be used to avoid extremely high discharge pressures which would be necessary with only one pump