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Non-return (or check) valves process specification 3

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sheiko

Chemical
May 7, 2007
422
Hello,

I am a french process engineer working in an Engineering Company.

My problem is that the piping engineer asked me the following data for a number of non-return valves:

- The min. and max. flowrates
- The min. and max. operating pressures
- The min. operating temperature
- The minimum pressure drop (for opening)

It is the first time i am asked this but i have found some data. For example, i have considered the design conditions for the max. conditions (flowrate, pressure). However i have some doubts concerning the definition of the min. conditions and the min. pressure drop.

Before asking the piping engineer more precisions, i was wondering if you could help me clarify some points, namely:

1/ Does the min. flowrate correspond to the min. operating flowrate or the min. flowrate for full lift?

2/ What is the purpose of min. pressure? Can i consider the atmospheric pressure (0 barg) or the full vacuum pressure (-1 barg) which could occur during start-up, shut-down or maintenance?

3/ What is the purpose and how to calculate the min. pressure drop? are there some typical values by type of valve?

If you have interesting references, please feel free to share.

Thanks in advance.


"We don't believe things because they are true, things are true because we believe them."
 
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Min Flowrate: 0.

I'll let others handle the other bits, now that I've shown the way!
 
I am only guessing but I think that the Piping Material Engineer is asking you the minimums and maximums for the line or service so he or she can select the proper valve so your process will be a success.
 
Dear sheiko,

I am a piping material engineer.

Followings are my answers,

1. min. operating flow rate
2. this question is for the mechanical design of valves.
therefore you should give him/her the most severe
condition. Then valve vendor will consider it.
3. Why did piping guy mention "(for opening)"?
If he wants the min. pressure drop for opening,
that is for the cracking pressure, which is the minimum
required pressure for valve opening.

I guess this valve is not normal check valve.....right?
 
Hello Sheiko,

I think your problem and our problem is that we do not exactly know what your piping engineer is looking for. Does he want to have information regarding the valves OR information regarding the process?

1) Regarding the minimum flowrate I would say that the process is prevailing. Your system will determine the flows going through a valve. The valve suppliers just have to make sure that they can handle the required flowrates. Do not use the flowrate for full lift, since this is different for each valve type/make.

2) Again this is a process question. You determine the pressure range you want. Minimum pressure is normally atmospheric pressure. But if your process will have close to vacuum pressures you should define that.

3) Depending on your system you can determine the maximum allowable pressure drop. Based on the available upstream pressure and the required pressure downstream in your process. You should probably do this together with the piping engineer, because your piping, bents etc. will influence this. The next step is to find a valve design that has less pressure drop than the maximum allowable pressure drop. A different valve design or valve make will have a different pressure drop. In general an axial flow non slam check valves will have the lowest pressure drop. Normally the valve suppliers will calculate the pressure drop for you if you give them the process conditions (flow/medium type/medium density/pressure/temperature).

You are the person that determines the conditions that your check valves has to comply with. The check valves do not determine your system.

Good luck,
Terje
 
Thanks all,

Do you agree that Min Flowrate always equals 0? is it sensible if we consider the min. operating flowrate?

"We don't believe things because they are true, things are true because we believe them."
 
Most or all check valves can handle a minimum flow rate of 0. But I think that your process determines the minimum flow rate.
 
In my opinion terje61 has the best answer yet.

Following comments as most important data for selcting a type and size of a check valve.

A check valve will give a pressure loss, a larger loss if the valve is not fully open. The pressure loss will vary from type to type on a large scale even if full open. Some type of check valve will give 'gulps' of fluid and slam back if too large sized (Typical for ball check valves). Other types (nozzle check valves spring assisted for closing, or some types disc valves spring assisted) will in almost every case prevent this.

Some types of checkvalves (for instance nozzle check valves)will perform with lower loss on a higher speed than others, and could preferably be installed direct on pumpsize outlet (after necessary straight stretch), before enlargement of pipesize.

You could also for other locations land out with a checkvalve below the normal pipeline size, just as you can with a regulating valve, in this case to minimize flow loss and pressurepeaks when closing.

If you have this in mind, sensible answers would be:

a) Min flow corresponding to the normal lowest flow in your pipeline above zero. (The valve would have to open, and keep open at this rate, but not necessarily at minimum pressure loss, but acceptable for process)

b) Normal flow. prevailing flowrate (Where the requirement is fully open valve, at minimum pressure loss range for the valve.)

c) Max flowrate in your pipeline. (Where the valve is designed for this, and a highrated valve may be downsized and placed near pump outlet equal to pumpoutlet size)

d) Cracking pressure. (Consider both horizontal and vertical up flow direction when evaluating valves later on)

e) Max operational pressure, including pressure peaks (To determin pressure class)

f) Minimum operating pressure to keep the checkvalve dropthight closed for backpressure. ( At for instance water basins this is essential, if not taken into consideration when designing piping. Requirement for a typical valve could be for instance 3 meter, if design of basin gives 2 meter or below, you will then have a problem.

g) Minimum temperature: for normal applications this would be zero centigrades (frost free), below this is only relevant if fluid or surroundings is expected for operation to be below zero, then minimum fluid and minimum surrounding temperature should be stated.

Other design/selection criteria: Valve to be of good quality, material, surface coating, standards suitable, selected on basis of lifetime dependability and minimum pressure loss, minimum pressure peaks (non slam closing?) and lowest dropthight pressure, preferred type if any, necessary sertification if any, price/performance evaluation on lifetime basis.

 
Your piping engineer could well be wanting to undertake a surge analysis. The performance of the check valve could well impact on the system response. In which case the engineer would need to reverse velocity vs deceleration requirements.

 
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