NRV's and FO upstream and down stream diameters 1

[mrtangent2]

ive recently come accross some specs internally that say

> FO's you need 5 upstream and downsteams. (not the flow meters) the standard FO's for pump min flow recycles on pumps.
I've not seen this before as 20+ years as a process engineer, has apparently its due to pipe errosion has anyone actually seen an issue with this for non -compliance ?

> NRV's I've seen an internal spec for 2 D downstream and 5D upstream - again ive never seen this and seen hundreds of NRV's just on valves and near bends and i'm not aware of a single failure or failure report - is this a valid reason..

Looking for a general perspective on this

 

[hacksaw]

The piping systems (BFW for example), I've worked around did not have formal restrictions other than maintenance access.

Elbows in close proximity to flow orifices meters affects the measured d/p not the flow or net pressure loss to an extent.

 

[LittleInch]

Is an FO the same as an RO (Restriction Orifice).

Flow measuring orifices, yes you normally need 5 u/s and 10 D/s or maybe the other way around.

FOr an RO you can get very high veleocities in the jet which take a few pie diameters to calm down.

NRV's - now thats a new on to me.

The problem with any internal company spec is that something happens in one place for a very particular reason, but then the "solution" is then imposed everywhere.

So I can see how something like a swing check which has low flow could have a high velocity stream d/s which in the right combination of material, fluid and posisbly even solids could cause erosion.

But commonplace? Not for me.

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

 

[mrtangent2]

Thanks for info.

FO i mean restriction orifice not flow meter.

I can understand FO in solids or slurry duty having downstream critica for pipework damage but not say on clean water.

 

[LittleInch]

Yes, but that's the problem with someone writing specs. To keep it simple they just lump everything and every fluid into one catch all requirement or recommendation....

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

 

[MJCronin]

Agree with LittleInch..... Far too many of these "thumb rules of piping design" come from tribal knowledge someone gleaned at their last employer ...

Based on a survey of the questions asked, this tribal knowledge issue seems endemic in the third world

The problem with any internal company spec is that something happens in one place for a very particular reason, but then the "solution" is then imposed everywhere.


Amen, brother .... Amen ....

Cheng is a company that sells "anti rotation flow devices" .....

http://www.chengfluid.com/flow_problems/check_valves

Regarding Swing-Check valves, Cheng states:

Flow disturbances emanating from the upstream elbows cause problems in the check valve. Most manufacturers of swing check valves recommend up to 10 diameters of straight pipe before the inlet of their valves. They also point out that swirl due to out-of-plane elbows will have a detrimental effect on flow and maintenance. Flow separation with shed vortices from the elbow and trailing edge of the valve disk, as it falls back into the stream due to pressure fluctuations, creating noise. The valve disk oscillating back and forth in the flow also sets up a stereo speaker-like pressure fluctuation. Reduced average flow through the valve will result from these elbow flow distortions. Another effect are pressure spikes that occur in the flow to the check valve due to localized matching of the separated flow signatures from the elbow and valve disk which will cause increased valve wear and pipe vibration amplitudes.

My own opinion is that if there is a flow disturbance/vibration issue of this magnitude, a swing check may not be the best choice ... there are other options

There is also this somewhat sketchy Eng-tips link to a chattering quick closing steam valve ...... Not sure that this is relevant

https://www.eng-tips.com/viewthread.cfm?qid=473130

Anyone have any other hard vendor evidence about straight piping design adjacent to NRVs ??

MJCronin
Sr. Process Engineer

 

[bimr]

The reason for the straight pipe upstream and downstream of flowmeters is to improve the accuracy of flow meters. The flowmeters still work with less straight pipe, but with less accuracy.

For typical fluid applications, there is no requirement that NRV's and FO have upstream and downstream straight pipe.