Steam flow rate calculation for boiler piping & PRV valve 2

[Asisraja D]

Hi Professionals
First of all thanks for all of you for helping me continuously.
We have installed 5 TON & 2 TON (Firetube boilers) in our new plant.

Boiler specifications :
Type: Fire in tube
Working pressure: 10 kg/cm2 (Both 5 TON & 2 TON)
Steam : Wet steam (saturated steam)

i have some doubts on the steam flow rate on main header and velocity for the PRV selection for our sub-header line.

Header size : 6"
Pipe Grade: ASTM A 106 GR.B

consider only one boiler is running at the time and another is standby if calculation purpose please take 5 TON is running.

so my doubt is
1.)how to calculate flow rate based on our pipe size & what about the steam velocity can we get to in header ?
2.)how much flow rate can we get if i install PRV (Pressure reducing valve) to reduce the pressure from 3.5 kg/cm2 to 2 kg/cm2 in DSP Bay ? ( i have shown it in the P&ID )
3.) why shouldn't i go for PRV instead of PRS (Pressure reduction station) to reduce the line pressure from 6 kg/cm2 to 3.5 kg/cm2 in DSP Bay ? ( I have shown it in the P&ID)?

i have attached the P&ID of the plan, if any misunderstanding happens by me please guide me in this. (i have 3 years of experience in projects so please bear me)

Thank you all.

 

[katmar]

I am unable to download or view your P&ID.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[Asisraja D]

Link

Sir can you please try this google drive link to download the file.

 

[davefitz]

Your local boiler code should have instructions on sizing and placing a pressure relief valve, usually called a safety valve. A boiler's safety valve needs to be installed in direct communication with the steam drum and not downstream of an outlet pressure reducing valve. The sizing of the safety valve should be based on the maximum steam capacity fo the boiler at its highest design pressure while firing its best fuel. If the boiler fires coal today but will be converted to oil or natural gas firing in the future then its steam flow capacity would likely increase so the safety valve sizing would be for the steaming ratewhile firing the best fuel.

The downstream lower design pressure process or piping needs to be protected with a second set of safety valves , located at the outlet of the pressure reducing valves. The sizing or capacity of these safety valves needs to be based on the pressure reducing valve's maximum flow capacity under the situation where the upstream higher pressure boiler is at its maximum design pressure and the pressure reducing valve has failed open.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[Asisraja D]

davefitz sir
Actually the p&I'd doesn't included safety valve but we have installed it. This is conceptual drawing so I excluded the essential components and my doubt is whether I can gor for PRV or PRS for pressure reducing purpose in branch connection. I am bit confused in these both terms.

And I really don't know why we always install PRS instead of PRV because the PRV cost is cheap and takes less installation place so please clear me in this.

 

[georgeverghese]

The diagram says the 100NB line to DSP bay is operating at 3.5kg/cm2, but there is no PRS shown on this 100NB line to reduce the pressure from 6kg/cm2 to 3.5kg/cm2. Pls explain.

Typically, we use PRS / PRV for reducing pressure for operational use. PSV is only for safety release. In a PRS or PRV setup, flow is smooth and constant. Through a PSV, flow is not all smooth or constant, it may be popping and not constant.

Why do you say PRV is cheaper than a PRS unit ? Can you attach a picture or sketch of the differences ?

 

[Asisraja D]

georgeverghese sir
The 100NB DSP bay line pressure is 6kg/cm2 only it was my mistake in the drawing.

Yes is we have ordered PRS and it costs us 2.7 lakhs for one PRS station.

The same I asked PRV for then it cost around 75 thousand
only.
Link
Please find the link for PRV I am mentioning.

 

[georgeverghese]

Can you post this without having to request access to "Google Drive " ?

 

[Asisraja D]

georgeverghese sir
I'm really sorry sir. I have updated the link now you can view it without any restriction. Thanks for trying to help me sir.

 

[georgeverghese]

So these PRVs' typically show poor control - considerable droop. Setpoint is not maintained as flow increases. On a 100NB line, assuming you have a 3inch PRV, we see from the graph that Cv = 600 approx for inlet pressure of 105psig/ exit pressure of 50psig.

Is this adequate for your use? What Cv do you need for this DSP bay at full flow ?

Pls note that with this type of valve, control at high turndown can be unstable also (ie bad control of setpoint during low flow).

Am not a field instrumentation expert - there may be more to say from others on E-Tips with more experience.

What is your company's reason for going with PRS at higher cost ?

 

[Asisraja D]

georgeverghese sir
I am not good some basics and I don't know what is Cv ? Please help me for selecting PRV for 40Nb filtered steam line purpose. In 100NB DSP bay we don't have any PRS because our MD told us to not install any pressure reducing station there.

I want help in selecting PRV for 40NB line and my vendors told us to go for 50NB PRV for filtered steam line due to flow capability.

Does this work here ?

Can I install 50NB PRV in 40NB line for getting enough flow ?

 

[LittleInch]

Asis,

so my doubt is
1.)how to calculate flow rate based on our pipe size & what about the steam velocity can we get to in header ?
You know your mass flow rate in the header at least (5,000 kg/hr). Steam tables will give you the density of saturated steam at your outlet pressure (6 kg). You know the ID of the pipe so it is a simple calculation to find out what the volumetric flow rate and hence the velocity is.

2.)how much flow rate can we get if i install PRV (Pressure reducing valve) to reduce the pressure from 3.5 kg/cm2 to 2 kg/cm2 in DSP Bay ? ( i have shown it in the P&ID )
You will get the flow possible when the valve is fully open. That is the valve CV that george talks about.


3.) why shouldn't i go for PRV instead of PRS (Pressure reduction station) to reduce the line pressure from 6 kg/cm2 to 3.5 kg/cm2 in DSP Bay ? ( I have shown it in the P&ID)?
The two terms are similar but depends on how YOU define the two. By the look of it a PRS as drawn has two parallel valves so might be sized at 2 x 100% or 2 x 50% - we don't know. It also has more isolation valves to replace one valve if required, but keep steam flow going. That's why it is more expensive.

Those PRVs you link look pretty basic to me.

Yes you can install a bigger valve, but valves are usually smaller than the lines, so it looks like you want too much flow in your 40 NB pipe.

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

 

[Asisraja D]

LittleInch sir

1) i calculated the steam slow rate as you told and it gave me 25 m/s steam velocity in 6" header.

2) You will get the flow possible when the valve is fully open. That is the valve CV that george talks about.
As you told i can get maximum flow rate if valve is fully opened but here the small correction i have already told that we are not going to install any PRV for 100NB branch if so can i take the velocity as 25 m/s as same i am getting in 6" main header. can i consider for this velocity for 100NB pipe flow rate calculation ?

By the look of it a PRS as drawn has two parallel valves so might be sized at 2 x 100% or 2 x 50% - we don't know

i could not understand the bolded fonts sir please explain me little bit.

i have attached the images of our received PRS Station at site in the link and some handsketch of pipe sizes included.

Link

Link

 

[LittleInch]

To get the velocity in the smaller pipe you need to know mass flow in that pipe. That information you haven't stated.

Your PRS looks a bit more like it, but I hate bypass valves.

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

 

[Asisraja D]

LittleInch sir

Thank you for sharing your knowledge and I don't know about the mass flow passing through 40Nb line.

So it can be measured by only through flow meters or else can I try any equations like continuity or Bernoulli ?

 

[georgeverghese]

"I want help in selecting PRV for 40NB line and my vendors told us to go for 50NB PRV for filtered steam line due to flow capability."

So how did these vendors size this PRV for you ? What flow did they use?

 

[Asisraja D]

georgeverghese sir
They just gave us information based on their subjective opinion in PRV selection but there is no any evidence for flow rate for 50Nb PRV to 40Nb line.

So it is their recommendation for us.