The hard road to our competitive edge, Part II

In Part II of his Lessons Learned column, Béla Lipták describes one of the many small steps process control professionals should take on the difficult, but necessary road to recapturing our competitive edge.

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 By Béla Lipták, PE

T

he lifetime operating cost of a pumping station is about a hundred times its initial cost, because pumping is a very energy-intensive process. A great deal of competitive leverage can be created by driving cost out of production through process optimization. Pump optimization is a great place to start because the returns on pump optimization can be substantial. In Part II we continue the discussion. (See Part I, The Hard Road to Our Competitive Edge.)

A pumping system is optimized when it meets the demand for the distributed liquid with the minimum investment of energy. When the pump station feeds a distribution header, and from that header each user meets its needs by throttling its own control valve, the total energy use will be minimized when the pressure drop across all user valves is at minimum. This occurs when the openings of all user valves are maximized and they are maximized when their inlet pressure is minimized.

Therefore, in such an optimized control system (Figure 1) the valve position controller (VPC-02) keeps lowering the set point of PC-01 and thereby, keeps opening all user valves until the most open valve reaches a 90% open. In addition to saving energy, this control system also guarantees that no user can receive less distributed fluid than required, because no user valve is ever allowed to reach 100% open.

FIGURE 1: ENERGY MISER



This control system minimizes energy and reduces valve maintenance.


Parallel Pumps
When multiple pumps operate in parallel, their combined head-capacity curve is obtained by adding up their capacities at the same discharge head. The total capacity of multiple constant-speed pumps is found at the intersection of their combined pump curve with the system curve. The same is the case with multiple variable-speed pumps, except that they have a pump surface instead of a pump curve, because velocity is also a variable and it adds a third dimension to their “personality.”

The pump station in Figure 1 can consist of two or more variable-speed pumps. When only one pump is in operation and the pump speed approaches 100 %, the pump speed switch (PSH-03) is actuated and the second pump is started by interlock #1. When both pumps are in operation and the total flow demand drops to 90% of the capacity of a single pump, the second pump is stopped after a time delay. This 0–30-in. delay (TD-04) guarantees that the second pump will start only if the drop in flow demand is permanent and therefore the pump is not cycled on and off.

Stopping/starting
Figure 2 (see below) illustrates that when multiple pumps (A&B) operate in parallel, their combined head-capacity curve is obtained by adding up their capacities at the same discharge head. The total capacity of multiple constant-speed pumps is found at the intersection of their combined pump curve with the system curve. The same is the case with multiple variable-speed pumps, except that they have a pump surface instead of a pump curve, because velocity is also a variable and it adds a third dimension to their “personality.”

The top portion of Figure 1 shows the system curve and pump curves. As the capacity demand rises, the speed of the operating pump increases until (at point A) it reaches 100% and PSH-03 starts the second pump. However, if, at the time of starting the second pump, the speed-control signal was unchanged, an upset would occur, because the speed of both pumps, and therefore their discharge pressure, would instantaneously jump from point A to point C. To eliminate this temporary surge in pressure, PY-03 is provided, which drops its own output signal to “x” which value corresponds to the required speed, when two-pumps operate at point A.

Therefore, when interlock #1 is actuated, the low signal selector selects signal “x” for control and thereby provides a smooth transition. After the switching, the output signal of PY-03 slowly rises up to full scale and consequently, as soon as it exceeds the output of PIC-01, the low signal selector returns the control back to PIC-01.

Once both pumps are operating smoothly, the next control task is to stop the second pump when the load drops below the capacity of a single pump. This switching is controlled by the low flow switch FSL-05, which is set at 90 % of the capacity of one pump (point B on the systems curve).

Using the coordinates of flow, pressure and speed, pump characteristics can be described by a three-dimensional surface and the process system the pump serves can be described by another surface. When multiple variable-speed pumps operate in parallel, their characteristic surfaces are summed in the direction of the flow coordinate to obtain their combined characteristic surface.

The operating line of a multiple-pump pumping station which is serving a particular process can be found where the two characteristic surfaces meet. A power consumption surface corresponds to the operating surface of the pump or pump station. When the pumping station consists of a large number of pumps (both constant and variable speed), the goal is to meet any load with a combination of pumps that will consume the least amount of power.

FIGURE 2: COMBINED HEAD-CAPACITY

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