Minimum Flow Recirculation Line 3

[kya]

I need to design an orifice size for a minimum flow recirculation line for a pump. I am aware on how to calculate the orifice size but need help on deciding parameters. I know the minimum stable flow rate and corresponding head on the pump from the vendor curves. What pressure differential should I use on the orifice sizing?

1- Use the minimum stable flow head

or

2- Use:
the max output head of the pump - orifice pressure differential = minimum stable flow head
and solve for the orifice pressure differential


Any resources/guidelines on minimum stable flow calculations related to an orifice for a pump would be appreciated.

Thanks.

 

[LittleInch]

3 Use an ARV instead...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

Take LittleInch's advice.

Is the driver constant speed, or variable speed. Do you have a constant head system curve, or is it variable. Each can make a big difference in the method of sizing an orifice plate for minimum flow.

Publish your pump and system curve and we might be able to tell you more.



Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[pierreick]

HI,

https://whatispiping.com/automatic-recirculation-valve/

Best option.
Pierre

 

[shvet]

Any resources/guidelines on minimum stable flow calculations related to an orifice for a pump would be appreciated.

https://www.cheresources.com/invisi...e-process-flow-schemes-for-centrifugal-pumps/

You should calculate several cases for hydraulic loops during min\norm\max pumping.

Please clarify what is a problem - how to calculate pressure drop across artifice? Or hydraulic loop? Or flowrate required? Or energy losses during normal operations? Or many others?

 

[MJCronin]

Agree with LIttleinch ...Is this a boiler feed pump ? ....

For HP pumps similar to a multi-stage boiler feed pumps, the pump vendor can supply the recommnedation for the ARC/ARV valve... i would make use of his experience


MJCronin
Sr. Process Engineer

 

[shvet]

@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.

 

[MJCronin]

Great Post shvet !!! .... An important guideline on a topic that many people confuse !!!

MJCronin
Sr. Process Engineer

 

[razookm]

I agree that more information is required. Here I will try to list important considerations

1. It shall be ensured that The pump-rated flow can take additional flow to continuous recirculation through the orifice. The rated flow of the pump should be the maximum required forward flow + Recirculation through the orifice.
2. Orifice shall be designed considering the required minimum flow and differential pressure expected at minimum recirculation conditions.
a. Inlet pressure at the orifice is the corresponding head at the minimum flow from the pump curve - pressure drop in the line from pump discharge to orifice inlet.
b. Outlet pressure at the orifice shall be calculated considering vessel pressure, head due to static height, and pressure drop in the line from orifice to vessel.

 

[taegyun2kimdhi]

Translated with

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(free version)

In most situations, ARCV works well, but if you have a high flow pump or work long time at MIN. FLOW, then a detailed review is required.

This is because the ARCV pressure(or flow) setting is different in the open and closed conditions of ARCV. This is similar to how an SRV has a set pressure(cracking) of 10 barg, but the blowdown pressure, the pressure at which it starts to close, is lower than 10 barg.

This is because the area that the pressure of the fluid acts on depends on its condition, i.e. min. FLOW before the ARCV starts to close or does not close completely, reducing its lifespan to about 6 months, and vibration problems have actually occurred.

I have expierences those on CCPPs and TPPs over 300MW.

 

[Snickster]

I am not sure what you are asking. At allowable pump flow there is a corresponding delta P for corresponding flow on the pump curve. So you know the flow and the overall delta P beween the suction and discharge line at minimum flow. Design orifice delta P including recirculation piping for that flow. I agree ARV's or other controls are better but a lot more expensive. If you can get by with a cheap fix and system is not that critical then go for it.