11 replies to this topic

[CHEMeng88]

Hi I am trying to size a control valve which is placed on a pump recycle line.

I am having some difficulties here. Looking at the data sheet, there is minimum flow, normal flow and max flow which corresponds to the minimum Cv, normal Cv and max Cv to be filled.

My question here is, how do i know what to set as flow rate for sizing this control valve which is placed on the recycle line from a centrifugal pump?


Appreciate your help.

[Dacs]

You have to approach the problem with the ultimate purpose of having that line in the first place.

It's there for minimum flow recirculation for pump protection (which we'll assume at 30% for discussion purposes).

From there, you can set the normal flow of your recycle FV. Maybe add 10% for rated flow and take 50% of normal for your minimum flow.

Do hydraulics to obtain the pressure upstream and downstream of your FV to get the needed delta P.

Cv's are easily calculated from above information at hand.

Hope this helps

[Mark-TR]

Hi Chemeng88,

First of all, you have to understand the flow requirements of your process unit, as stated above the ultimate objective of a pump recycle line is minimum flow protection.

Consequently the pump minimum flow shall be covered by the maximum Cv of your control valve. Check the pump curve according to pump vendor, there you will find the minimum flow of the pump.

A good approach is to consider normal flow as 100% of pump minimum flow and maximum flow as 110% of pump minimum flow. Others may only define maximum flow of control valve equal to 100% pump minimum flow and no normal flow.

But be carefull with the minimum flow for control valve selection. Recycle control valve shall be designed to control flow also in turndown conditions.

Pumps minimum flow may vary from 60-30% of pump rated flow.

If the turndown of your process unit (or downstream system) is 50% of the rated flow and the pump minimum flow is 60% of the rated flow, your control valve minimum Cv shall be designed for 10% of the pump rated flow.

This may have big impact in the control valve selection.

kind regards

Edited by Mark-TR, 12 September 2012 - 10:56 AM.

[fallah]

CHEMeng88,

As you know minimum flow control valves in pumps recycle lines are normally closed and would be opened if the flow in discharge line reaches a prespecified minimum value. Nevertheless if minimum continuous stable flow reported by relevant vendor to be, let say "F", then your requested data of various flowrates is normally set as per follows:

1) Maximum flow rate=1.1*F
2) Normal flow rate=F
3) Minimum flow rate=0.3*F

Fallah

[TMQ]

Hi

I am having same problem in filling out my data sheet for 1" recycle control valve. With design criteria as follows


capacity : 0.975 m3/hr
Diff Head : 0.975 mlc
Driver : 4 kw

Client wants to put 1" recycle control valve for a glycol circulation pumps for start up. At present pumps don't have a recycle control valve from discharge to suction to be used during start-up, to prevent the pump to start from zero pressure to 8200 kPa which create pump belt loose or cut and end with pump maintenance. The start up is done manually and with 2 operators. One operators in DCS and the other operator in field to do manual recycle from discharge to suction. Also to reduce the risk of high pressure flow line to operator.

I don't have pump characteristic curve but have flow rates .


Automatic Glycol pump start FQISA-3501 (H/LL) is installed, a low low flow set @ 20.77 m3/d starts the non-duty Glycol Circulation Pump and the other Glycol Circulation Pump is stopped, the high flow alarm is set @ 21.86 m3/d

FQSI = Flow Quantity Speed/Frequency Indicating


Low low & High high pressure trips are installed for Glycol Circulation pump protection, PZA-3501/03 (LL) set at 100KPa (g) to prevent a low low suction pressure and PZA-3502/04 (HH) set at 9,000 KPa(g) to prevent a high high discharge pressure, the concerned Glycol Circulation pump will shut will shut down if any of the above are activated

Kindly guide for the above points.

1) Min, Normal , Max flowrates
2)Pressure Drop across valve (i presumed a globe valve)
2) Flash % wt (Don't know how to calculate)
3) Travel ?
4) Cv
5) Allowable sound Level

You help will be appreciated

TMQ

[TMQ]

Hi
Please read Diff Head as

Diff Head : 7667 mlc

TMQ

[breizh]

TMQ

Diff head : Number and unit are wrong!

You need to know the fluid and its properties or ask support from others .
Check the standards available at your workplace .

Breizh

Edited by breizh, 01 November 2012 - 12:12 AM.

[S.AHMAD]

Sizing of control valve for centrifugal pump minimum recycle line, the minimum and maximum flow is not important figures since the valve is shut most of the time. What you can do is to calculate the Cv using the pump minimum flow and divided the calculated Cv with 0.8 to obtain the required Cv. Then select a linear characteristic control valve that having Cv very close to the required Cv or slightly higher. This will gives valve opening of about 80% during normal operation of the valve.

Edited by S.AHMAD, 01 November 2012 - 02:21 AM.

[kkala]

Indeed, the method locally applied for minimum flow valve process data sheet is simplified, as follows.
1.Flow Q= minimum flow as advised by pump supplier (or assumed and checked later through supplier's data).
2.Upstream pressure = shutoff pressure of the pump (lowest suction pressure Ps is of importance, range is given in case of significant variation of Ps ); this even though the valve operating point considers e.g. Q=30% of pump rated flow.
3. Downstream pressure = calculated for downstream piping for flow rate Q.
4. Noting the specific pump, the minimum flow of which shall be controlled by the valve. As pump flow decreases, valve flow rate increases (reverse action).
5. Rest is undertaken by the Instrument Department. I understand Process Dept gives above data for normal Cv and considerations for max Cv, min Cv are made by Instrument Dept, in a way similar to the useful suggestions of above posts.
6. Mechanical minimum flow valves (e.g yarway type for BFW pumps or NH3 pumps) are provided by the supplier of the pump.

[TMQ]

Thanks for your reply

S. Ahmad . I have worked out Cv =10.5 and dividing by 0.8 it comes to 13.125 . Now can you suggest value and value opening (80%).

TMQ

[TMQ]

TMQ

Diff head : Number and unit are wrong!

You need to know the fluid and its properties or ask support from others .
Check the standards available at your workplace .

Breizh

mlc = meter liquid column
Diff Head number is right

[TMQ]

Sizing of control valve for centrifugal pump minimum recycle line, the minimum and maximum flow is not important figures since the valve is shut most of the time. What you can do is to calculate the Cv using the pump minimum flow and divided the calculated Cv with 0.8 to obtain the required Cv. Then select a linear characteristic control valve that having Cv very close to the required Cv or slightly higher. This will gives valve opening of about 80% during normal operation of the valve.

Thanks for your reply

S. Ahmad . I have worked out Cv =10.5 and dividing by 0.8 it comes to 13.125 . Now can you suggest value and value opening (80%).

TMQ