Metso’s Nelprof® Helps Select a Pump for Optimum Control Valve Pressure Drop
Figure 1. The control loop for which installed gain will be analyzed to select a pump that will result in optimum control valve pressure drop.
Three possible pumps for the system will be considered and the one that allows satisfactory controllability while minimizing energy consumption will be selected. Curves of P1, the pressure just upstream of the valve, are shown for each of the three pumps along with the power required by each at a normal flow rate of 400 gpm. These curves each slope downward in proportion to flow squared from the 100 gpm pump head (45, 60, and 75 psig respectively for Pumps A, B, and C) to a pressure 10 psi lower due to the combined effect of the 5 psi pressure loss in the upstream piping and the 5 psi decrease in pump head from 100 gpm to 600 gpm stated in the figure. The curve for P2, the pressure at the control valve outlet starts with the 10 psig static head of the tank at very low flows and increases in proportion to flow squared to 30 psig as the downstream piping and heat exchanger pressure losses increase to their 600 gpm values. The control valve pressure drops (the difference between P1 and P2) are indicated in Figure 1 by the arrows at the left side of the figure for 100 gpm and at the right side of the figure for 600 gpm. The analysis will be performed based on using a Neles R Series segment ball valve. Figure 2 shows the results of the Nelprof installed gain calculation.
Metso’s recommendations for installed gain (and the rules that the Nelprof program uses in making control valve recommendations) are:
Within the specified control range:
- Gain ≥ 0.5
- Gain ≤ 3.0
- Gain (max) / Gain (min) ≤ 2.0
- As constant as possible
- As close to 1.0 as possible
Other white papers that may be of interest:
Figure 2. Comparison of installed control valve gain for all three pumps.
With the 17 horsepower pump, besides requiring a more expensive 6 inch valve, the gain graph looks terrible. The installed gain is the highest of the three (meaning a larger flow error for the same valve position error), it drops to 0.4 at the maximum flow and the variation in gain over the flow range is 6.7 to 1, much greater than the recommendation of 2 to 1. This is large enough that it would be very difficult to come up with PID tuning parameters that would give both good and stable control over the entire required flow range. The gain graphs of both the 23 and 29 horsepower pumps fall within the recommended gain criteria, but the 23 horsepower pump is the winner, since its gain is more constant and closer to 1.0, and it is also the more economical of the two to operate.
Note that although Figure 2 shows graphs for more than one valve, this graph was produced by combining three graphs using graphics software. Nelprof only shows the graphs for one valve at a time. By clicking on different valves in the project tree at the left of the Nelprof screen you can instantly jump from the graph of one valve to another, making it very easy to compare the graphs of several valves.
A useful feature of Metso’s Nelprof is that in addition to valve parameter files specific to Metso’s valve products, it includes generic valve parameter files for the most common types of control valves, that allow the user to make calculations that do not favor any specific valve manufacturer.
Installed Gain as a Control Valve Sizing Criterion
Aerodynamic Noise in Control Valves
Valve Aerodynamic Noise Reduction Strategies
Determining the Pressure Drop to be Used in a Control Valve Sizing Calculation
Size Matters: Control Valve Sizing 101
The content of these white papers are just a small portion of what you will learn in Dr. Monsen's book: Control Valve Application Technology