Steam Pressure Reducing Stations 2

[Jonathon Huse]

Hi guys,

On a current project we are installing a new steam line and pressure reducing station to service a crude vegetable oil refining plant. The inlet pressure to the station is 10 barg (boiler pressure), and out of the PRV the pressure is 3 barg i.e. the operating pressure of the refinery/process plant equipment (heat exchangers).

The question I have is primarily around best practice guidelines for the installation of the PRV itself. My understanding to date was to ensure a certain amount of pipe diameters (straight) before and particular after the valve/pressure drop, to ensure laminar flow and accurate modulation. My understanding was to allow 2-4 diameters beforehand, and at least 10 afterwards of straight pipe. On the current Bailey PRV that the contractors have installed, they have left 10 before the valve, but only 4-5 after. After then reading the technical manual for this valve, this is actually the supplier recommendation. My questions I hope you guys can answer:

- Is it not more critical to have laminar flow after the PRV (i.e. minimum 10 pipe diameters) so as to ensure the pressure in the downstream/low pressure port is accurate? And is this not because turbulent flow causing vortices/eddy currents can create points of lower/higher pressure which if reaching the port, provide a false indication of line pressure and cause the valve to oscillate accordingly.
- Similar to the above, with the reduction in pressure and thus increase in velocity (kinetic energy), the pipe immediately on the discharge of the PRV should be 1-2 pipe sizes greater than the inlet size?
- The reason best practice guidelines commonly say only 2 diameters of straight pipe on the inlet is because the inlet pressure is not the critical control parameter compared to the discharge pressure with which the equipment operates?

Would really appreciate some guidance on this. The more detail into the fundamental engineering flow mechanics of it the better as i'm into that level of detail, even as a high level project manager. Have attached a photo of the contractors installation which I questioned during my site visit today.

 

[katmar]

You will never find laminar flow (Re < 2100) in an industrial steam line. The reason for having the downstream tapping point some distance away from the valve is to allow the velocity head at the vena cotracta to be recovered as pressure. See

https://en.wikipedia.org/wiki/Orifice_plate

Follow the manufacturer's installation guidelines.

Pipe sizes before and after the valve should be determined using standard criteria and not by following some rule of thumb.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[Jonathon Huse]

Thanks Katmar.

If you refer to the attached drawing though, the downstream tapping point is immediately at the discharge in this specific models circumstance, where the flow has just been throttled.

What are you referring to by 'standard criteria'? Our plant demand is 1500kg/hr, meaning we are Reducing from DN65 10 bar then dropping to 3bar at DN100. I would have suspected to run the DN100 immediately following the discharge to decelerate the flow. The PRV is size DN40, and they have run this immediately following the discharge. 1300kg/hr in a DN40 line throttles the flow at 3bar and I would have thought this would be detrimental to the PRV life

 

[katmar]

By "standard criteria" I meant the pressure drop and velocity constraints. 1500 kg/h at 3 barg sounds reasonable. Depending on the quality of the steam at the inlet to the PRV (I see that you have a water separator there) the temperature of the 3 bar steam could be as high as 162°C and this would give a velocity of about 24 m/s in the 100 NB pipe. But the velocity would be 160 m/s in the 40 NB section. I don't believe this high velocity (in the 40 NB section) will be detrimental to the PRV life, but it will be noisy.

With this type of PRV you set the pressure you want by manually adjusting the valve while watching a pressure gauge somewhere downstream. It looks like you have a socket for the pressure gauge in the 100 NB line more or less in line with the top of the relief valve. If you adjust the valve to give 3 bar here, you would find a different pressure at a pressure gauge mounted immediately after the reducing valve. The difference between the two gauges would depend on the pipe fittings and the distance between them. Unfortunately this difference will change with changing flow rates. The PRV will hold the pressure at its outlet constant and you will see that the downstream pressure varies as the flow rate changes. In practice this variation is usually small and can be neglected.

With your flow rates and pipe sizes I would not expect this to be a problem for the installation as in your photograph. Ideally you would want the 40 to 100 reducer as close to the PRV as possible, but this makes the isolation valves more expensive and your installation is fairly typical.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[LittleInch]

Jonathon,

I have a feeling you are mistaking "laminar flow" with steady flow or fully developed turbulent flow.

Laminar has a particular meaning within flow design whereas what I think you mean is "non turbulent" Flow immediately downstream of a control valve is full of swirls, eddies and changing velocities including rotational ones. As the flow enters a pipe section it relatively quickly looses these variable vector flows and becomes more regular in the flow profile. Similarly if flow has gone through a number of bends, valves etc just before it enters a control valve, this can affect the working of the valve, or cause wear in one location.

Severe turbulence can impact on pressure readings and cause fluctuations in those readings, hence the guidance that some length of pipe is left to allow the flow to stabilize before and after a control valve.

Katmars answer, as always, is precise, accurate and has years of experience behind it.

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

 

[Jonathon Huse]

Thanks to both of you. Why is it so common for straights to be a specified requirement downstream of the PRV as opposed to upstream? because of the typically higher line velocities on the discharge?

I think I understand what Katmar is saying. Typically the downstream pressure gauge is used to establish the PRV operating setpoint. While the pressure immediately out of the PRV may not be exactly 3.0 barg on the dot, we use the downstream pressure gauge located at a point where the velocity profile has had a chance to stabilise as an indicator of our line pressure. This sound correct?

 

[LittleInch]

Partly line velocities, but also the flow is very turbulent coming out of a control valve and takes longer to establish a more even velocity profile. (Note PRV can mean Pressure Regulating Valve or Pressure Relief Valve - better to spell it out to avoid confusion...)

In my Opinion your last statement is correct.

BTW, I assume the pressure relief exit is going to be piped somewhere safe?

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

 

[jph90]

Still in construction, we will be running the relief line out of the building the reducing station is in.

You guys have helped a lot, really appreciate it