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Aug
04

Effect of Mechanical Design – Pumps and Piping

The increasing emphasis on saving energy and capital costs can lead to pump and piping designs that inadvertently cause a severe loss of turndown in flow loops and a significant increase in noise , delay, and lag in temperature and pH loops. The deterioration in loop performance can create an insidious reduction in process efficiency and quality whose cost outweighs the original intended savings.

The key pumping and piping system parameter for final control elements is pump pressure. The pump’s total discharge head is often chosen without knowing the implications in terms of process control. The reduction of available pressure drop for a control valve and even the elimination of the control valve has been a favorite target for energy savings. Typically loss of controllability is not considered in these decisions.

The minimum controllable flow for variable frequency drives (VFD) depends more upon static head than cogging at low speeds. If a conscious effort has lead to a low frictional loss piping system, the use of speed to control flow will become erratic as the total discharge head approaches the static head. The flatter the curve and the greater the slip or speed variability, the greater the flow variability at low speeds. Of course the worse possible case is where an excursion in downstream pressure causes the static head to exceed the discharge head leading to reverse flow. In one application a rise in reactor pressure lead to reverse flow of reactants and catalyst through the pump into the reactant feed tank, a hazardous besides damaging condition. Torque control can help but a frictional loss is still needed to provide cushion and turndown. For more information on the effect of pump curves on VFD checkout “Watch out with variable speed pumping” in Chemical Processing.  For the impact of turndown see the Control Talk column “Downturn, Turndown
and the “Top Ten Limitations – Turndown

The minimum controllable flow for a throttling valve depends upon the amount of pressure drop available to the control valve as a percent of the system pressure drop. Since valves are targets for saving on pumping costs, some valve manufacturers may say you can allocate only 5% of the system drop to the valve. This is true if you don’t need any turndown. The old rule of thumb was 50% which provides a reasonable but not fantastic turndown. As you allocate the less pressure drop for the valve, the installed characteristic becomes more distorted and the minimum flow increases as determined by valve stick-slip that is greatest near the shutoff position. Also, the maximum flow decreases as you run out of pressure drop available for the control valve. Chapter 7 on Fundamentals of Final Control Elements in “Essentials of Modern Measurements and Final Elements in the Process Industry” discusses and quantifies distortion of the valve characteristic and loss in the rangeability.

To prevent flashing in the pump, the suction head must be greater than the minimum NPSH of the pump and the piping system pressure and the minimum pressure in the valve vena contractor must stay above the vapor pressure of the fluid. When flashing is unavoidable, the valve is located at the vessel entry so that bubbles do not collapse in the valve or piping causing cavitation damage.

The key pumping and piping system parameters is length and velocity for flow, pH, and temperature measurements. To minimize the noise in differential head and vortex meters, the straight length must meet exacting criteria. The distance between a static mixer and a pH electrode and between a heat exchanger and a temperature sensor should be at least 20 pipe diameters to insure mixing. The increase in transportation delay (e.g. 2 seconds for 10 feet of 6 inch pipe at 5 fps) is less important than the decrease in noise. For a desuperheater the distance from the outlet to the temperature sensor must provide a residence time of at least 0.2 seconds and a velocity of at least 25ft/sec to insure a collapse of bubbles. Velocities above 350 ft/sec can cause vibration. For a further discussion on these requirements, check out the Control Talk column “Straight Talk

Velocities of 5 fps or more are critical for keeping pH electrodes clean in fouling applications. The higher velocity also significantly reduces lags by an increase in mass transfer rate of ions. There is a similar advantage for temperature sensors in terms of keeping them clean and more responsive from an increase in the heat transfer rate with velocity.

Gravity flow systems may seen attractive from energy and capital costs, but the increase in transportation delay magnitude and variability can be huge. Now a change in valve position does not immediate translate into a change in flow at the destination as it does with a pumping system. Instead we have a wave traveling a few fps or less and in the case of a dip tube a falling film.

Don’t get me started on dip tubes. They are principle source of deadtime in neutralizers. Standard mechanical design practices can lead to reagent injection delays of minutes to hours killing a pH control system. Stay tuned for the top ten mistakes made in a pH system design.

In the meantime you can get more information on the effect of pump and piping systems at “Secret Installation Effects”, “Location, Location, Location – Part 1”, & “Location, Location, Location – Part 2

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