@kya
For info. Chevron doc. PMP200-102
273 Flow Control Methods
Direct flow measurement and control is typically used to maintain minimum flow rates and to prevent pump runout. Advantages are:
1. Better accuracy than pressure control because pump flow is measured directly.
2. Flow control is unaffected by fluctuating suction pressure. It is effective on pumps with flat or drooping curves or parallel pumps with dissimilar performance curves.
Flow control usually costs more than pressure control, unless flow is to be measured for other process reasons.
Methods of flow control include using a bypass with one of the following: a globe valve (or orifice), an automatic recirculation (ARC) valve, proportional control using a flowmeter, proportional controller/control valve, or a snap-acting solenoid valve. Runout can be prevented by either a self-contained (pilot operated) diaphragm valve or proportional controller/valve arrangement, mounted in the pump discharge piping.
1. Fixed recirculation through a globe valve (or orifice). See (a), Figure 200-53.
A globe valve or orifice is mounted with or without a meter in a bypass line from pump discharge back to the storage vessel or tank.
This is an expensive approach. Energy losses occur across the orifice or valve. Continuous recirculation should be specified when pumps are purchased to ensure sufficient capacity for both the process flow rate and the bypass flow rate. If continuous recirculation is added as a retrofit, the pump may tend to operate too far near the right end of its curve.
Due to the energy costs, this is only recommended for low-energy services (less than 10 HP).
Note that the globe valve could be completely closed, leaving the pump unprotected.
2. Automatic Recirculation (ARC) valves. See (b), Figure 200-53.
ARC valves are often used for flow control, primarily with Sundyne highspeed, integral-gear pumps. These valves discharge a side stream back to suction when flow falls below the recommended minimum. Because ARC valves recirculate only when they have to, they minimize energy loss. However, ARC valves are only available in ANSI classes 150 and 300 and are costly.
ARC valves are most economical with high bypass flow rates and larger differential pressures. Under these conditions, recirculation with either a fixed restriction or globe valve bypass becomes too expensive due to energy losses.
ARC valves can also be used when a pump curve will not allow the additional flow required to satisfy the process and a fixed-recirculation bypass.
274 Proportional Flow Control
A flow controller receives a signal from a flow sensor/transmitter and positions a control valve located in the bypass piping. The control valve stays closed as long as the pump operates above minimum flow. It begins to open at minimum flow and bypasses just enough to keep the pump running at exactly minimum flow.
This type of proportional control is more expensive than using an ARC valve because of the instrumentation involved, but may be feasible when a meter run is already installed for other reasons.
Proportional controls can also be used to prevent runout (see (f), Figure 200-53).
The control valve is mounted in the pump discharge line rather than the recirculation line. The flowmeter/controller senses excess flow and positions the control valve to reduce flow to an acceptable rate.
On-off control with a flow switch and solenoid valve. See (d), Figure 200-53.
A flow switch in the discharge piping detects low flow and opens a bypass-mounted solenoid valve. Flow is routed back to suction as long as the process flow rate is low enough to keep the flow switch tripped. If the bypass line allows too much flow to be recirculated, a fixed restriction orifice should be installed.
The flow switch opens only when minimum flow occurs, avoiding the potential problem of operating near the end of the curve and energy losses associated with a continuous recirculation.
Flow switches can also be used for high and low flow alarms and shutdowns.
275 Self-Contained Flow Control Valves
A self-contained diaphragm valve can be used to prevent runout. This type of valve works by sensing differential pressure and using this pressure to modulate the position of the trim and diaphragm. A disadvantage is that the valve has small diameter passages that may become plugged with sand and scale. When this happens, the valve will operate erratically, and can worsen the pumping problems. For this
reason, do not use diaphragm valves in sandy or dirty service (such as on water wells), or in services that could lead to corrosion or scaling of the internal trim.
Avoid hot services that could lead to failure of the elastomer diaphragms.
Self-contained flow control valves can be used to prevent runout as shown in (e), Figure 200-53 or can be used in a bypass line back to suction to keep flow above minimum.
276 Economics of Flow Control
The economics of minimum flow systems depend on the bypass flow rate, differential head, and the equipment already in place:
1. Fixed-orifice recirculation is inexpensive and probably the best method at low flow rates and low differential pressures. It uses only a valve or orifice with or without an indicator, pressure gage or rotameter. However, it constantly wastes energy across the valve or orifice and can be expensive to operate with high head pumps or with pumps that require a large minimum flow rate.
2. Flow control through power measurement has the same problems inherent in pressure control. Except for applications involving very low flow rates and pressures, this method may be the least expensive. This method is suited for retrofitting existing equipment because no piping changes are required and pump operating conditions do not change.
3. On-off bypass control using flow switches and a solenoid valve is simple and relatively inexpensive. It does not waste energy and does not allow the pump to operate at the end of its curve.
4. ARC valves and proportional controls are costly. Restrict their use to critical, unattended, unspared, high-head pumps or pumps with large minimum required flow rates.
