Controlling multiple pumps operating in parallel or series
Often, an application or process calls for multiple pumps operating within a piping network. Pump load, such as flow rate or pressure, is shared between these multiple pumps. The present disclosure relates to effective means of distributing the pumping load in a manner that satisfies the process requirements while keeping the pumping machinery safe from functioning in damaging operating regions. It also discloses a method of operating pumps in an efficient or optimal fashion. An additional aspect is a method of using an open-loop response to deal with large transients threatening to force a pump into an operating region that might result in damage or destruction.
1. A method for controlling a pumping system comprising a plurality of variable-performance centrifugal or axial pumps, each having a Minimum Continuous Stable Flow control limit, pertinent instrumentation, and a control system, the method comprising manipulating a performance of the pumps, such that all pumps arrive at their respective Minimum Continuous Stable Flow control limits approximately simultaneously as a process flow rate is reduced.
2. The method of claim 1 , wherein signals from the pertinent instrumentation are used to scale a pump map for each pump, calculating a pump control variable, S, such that each pump's Minimum Continuous Stable Flow control limit has a value equal to the Minimum Continuous Stable Flow control limits of all other pumps.
3. The method of claim 1 , wherein the pertinent instrumentation comprises instrumentation for measuring a value related to a volumetric flow rate.
4. The method of claim 1 , wherein pump performance is changed by varying a pump's rotational speed.
5. The method of claim 1 , wherein pump performance is changed by varying a throttling valve's opening.
6. The method of claim 1 , wherein upon reaching a preset value of a pump control variable, S, a recycle valve is opened a predetermined amount as quickly as possible.
7. The method of claim 6 , wherein the predetermined amount of valve opening is variable during operation.
8. The method of claim 7 , wherein the predetermined amount of valve opening is based on the speed at which a pump's operating point is moving in the direction of zero flow.
9. The method of claim 8 , wherein the predetermined amount of valve opening is repeated at intervals until the pump returns to a safe operating region.
10. The method of claim 1 , wherein the plurality of pumps is controlled to achieve a desired balance when pumps are operating far from their Minimum Continuous Stable Flow control limits.
11. The method of claim 10 , wherein the desired balance results in a minimum total power.
12. The method of claim 1 , wherein the pump control variable, S, is calculated as:
S
=
K
H
Q
2
+
b
where Q is volumetric flow rate and H is head, while subscripts K and b are constants.
13. The method of claim 1 , wherein the pump control variable, S, is calculated as:
S
=
K
Q
N
-
b
where Q is volumetric flow rate and N is rotational speed, while subscripts K and b are constants.
14. The method of claim 1 , wherein the pump control variable, S, is calculated as:
S
=
K
(
Q
N
)
2
-
b
where Q is volumetric flow rate and N is rotational speed, while subscripts K and b are constants.
15. The method of claim 1 , wherein the control system comprises a master controller maintaining a main control-variable at its set point; and for each pump, at least one load-sharing controller maintaining a balance between all pumps sharing a pump load; each load-sharing controller being configured to equalize a distance from its pump's operating point to the Minimum Continuous Stable Flow control limit.
16. The method of claim 1 , wherein the Minimum Continuous Stable Flow control limit is a function of the pump's operating conditions.
17. The method of claim 16 , wherein the operating conditions, of which the Minimum Continuous Stable Flow control limit is a function, comprise a pump rotational speed.
18. An apparatus for controlling a pumping system comprising a plurality of variable-performance centrifugal or axial pumps, each having a Minimum Continuous Stable Flow control limit, pertinent instrumentation, means for manipulating each pump's performance, and a control system, the apparatus comprising means for maintaining approximately equal distances between all pumps' operating points and their respective Minimum Continuous Stable Flow control limits.
19. The apparatus of claim 18 including means for calculating a pump control variable, S, based on signals from the pertinent instrumentation, such that each pump's Minimum Continuous Stable Flow control limit has a value equal to the Minimum Continuous Stable Flow control limits of all other pumps.
20. The apparatus of claim 18 , wherein the pertinent instrumentation comprises instrumentation for measuring a value related to a volumetric flow rate.
21. The apparatus of claim 18 , wherein means for manipulating each pump's performance comprises means for varying a pump's rotational speed.
22. The apparatus of claim 18 , wherein means for manipulating each pump's performance comprises means for varying a throttling valve's opening.
23. The apparatus of claim 18 including a control system for opening a recycle valve a predetermined amount as quickly as possible when a pump operating point reaches a preset value of a pump control variable, S.
24. The apparatus of claim 23 including a calculation unit for calculating a varying value for the predetermined amount of valve opening during operation.
25. The apparatus of claim 24 including means for basing the predetermined amount of valve opening on the speed at which a pump's operating point is moving toward zero flow.
26. The apparatus of claim 23 including means for repeating the predetermined amount of valve opening at intervals until the pump returns to a safe operating region.
27. The apparatus of claim 18 including a control system for controlling the plurality of pumps to achieve a desired balance when pumps are operating far from their Minimum Continuous Stable Flow control limits.
28. The apparatus of claim 27 including means to balance the pumps for minimum total power.
29. The apparatus of claim 18 including a calculation unit for calculating a pump control variable, S, as:
S
=
K
H
Q
2
+
b
where Q is volumetric flow rate and H is head, while subscripts K and b are constants.
30. The apparatus of claim 18 including a calculation unit for calculating a pump control variable, S, as:
S
=
K
Q
N
-
b
where Q is volumetric flow rate and N is rotational speed, while subscripts K and b are constants.
31. The apparatus of claim 18 including a calculation unit for calculating a pump control variable, S, as:
S
=
K
(
Q
N
)
2
-
b
where Q is volumeu-ic flow rate and H is head, while subscripts K and b are constants.
32. The apparatus of claim 18 , wherein the control system comprises a master controller maintaining a main control-variable at its set point; and for each pump, at least one load-sharing controller maintaining a balance between all pumps sharing a pump load; the load-sharing controllers are configured to equalize a distance to the Minimum Continuous Stable Flow control limit.
33. The apparatus of claim 18 including means to calculate the Minimum Continuous Stable Flow control limit as a function of the pump's operating conditions.
34. The apparatus of claim 33 , wherein the operating conditions, of which the Minimum Continuous Stable Flow control limit is a function, comprise a pump rotational speed.