Help me in selection of PRS station for 400 Kg/hr steam boiler

[Asisraja D]

Hi professionals
We have purchased a steam boiler with capacity of 400 kg/hr at maximum operating pressure of 10 kg/cm2 and now we have planned to install a pressure reduction station. i will add some information below for better understanding.

1) Steam outlet nozzle size : 1"
2) Main steam line header size : 3"
3) PRS Station nozzles size : 1.5" X 2"

As the vendor suggest us to go for PRS station from 10 kg/cm2 to 3-4 Kg/cm2 for the header.

he just asked me a few questions to conclude this those are just
*how many branches you have planned for ?
*how much pressure you expect at the equipment side ?
* and he roughly says at 1" line you could have get 150 - 160 kg/hr steam flow rate.

my answer to that question is

we have planned for taking one secondary header (2") from the main header (3") then we planned for 6 branches at (1") nozzle.
we need 2 kg/cm2 at equipment side. (reactor jacket side)

can anyone please explain me a bit how exactly he could decide the factors here ?
i have attached the vendor quote and GA drawing of the boiler.

i need to some explanation for the conclusion made by him.


if anyone have few basics to make be clear on this please send me your valuable comments on this.


Thank you.

 

[goutam_freelance]

I think what the vendor suggests is that it may be more economical to run the high-pressure steam piping up to near the consumption point, which may reduce the piping cost to some extent. However, if you have 6 nos 1 in lines, the cost of 6 PRSs may be too high. In this respect, your design seems OK.

and he roughly says at 1" line you could have get 150 - 160 kg/hr steam flow rate.

This may be due to high steam velocity, almost 62 m/s with 3 kg/cm2 steam with a flow of 200 kg/h in 1 in line.

 

[TBP]

With one large control valve, you may get "hunting" on low loads. The control valve cracks-open, and immediately hits it setpoint, so the valve throttles-in. Except that it has only cracked open, so it closes fully. Now it's under the set point, so it starts to open - and immediately hits it's set-point, so it throttles-in, and closes. Over and over and over...

 

[LittleInch]

Well 95% of that "quote" is standard marketing data.

Despite saying its good to transport at a higher pressure, they are actually quoting you only one pressure reduction valve with a claimed 40:1 turndown, which is difficult to achieve in practice. The question being what is YOUR required turndown for continuous operation, not start up.

Also the standard diagram incorporates a manual bypass line which is very dangerous. Give someone a valve and they will open it one day....

But your pressures are all over the place.

The quote says 4 to 6 barg, you say 3-4 barg /(kg/cm2) and they you say you only need 2 bar?

You seem to be expecting the vendor to magically understand your system and do the design. Not unsurprisingly he doesn't know what's inside your head and also doesn't want to do the design for the entire system, he just wants to sell you a PRS package.

 

[TBP]

The vendor may be allowing for the pressure drop across whatever the temperature control valves are. But who knows? This is a small installation, so with several little steam connections, I suspect nasty little solenoid valves as masquerading as "temperature control valves".

And I fully agree on bypass valves. They have caused me FAR more problems than they have ever solved...

 

[georgeverghese]

Your boiler supplies 10barg sat steam, and your final users are running on 2barg steam, so you can expect this steam to be considerably superheated by the time it is dropped down to 2barg. Check what is the permissible superheat at 2barg at your reactor steam side heat transfer device ( some steam jacket or HX ??). If necessary, a desuperheater may be required - careful with the effect of flow turndown on desuperheater performance. Turndown affects the selection of type for the desuperheater.

 

[TBP]

My experience with steam pressure reductions in this range is that superheat isn't a practical issue. There's almost always some moisture level present in the steam, and that the bit of superheat you get after a pressure reducing valve is gone a few feet down the line - it just gives you better quality steam at the point of use.

 

[Snickster]

It is up to you to decide whether your piping is sized properly for the flowrate, pressure and temperature. According to the vendor he will provide you with a PRV to reduce the pressure from 10 kg/cm2 to 3-4 kg/cm2. The PRV should be sized for the required flowrate so that it is about 80 to 90 % open at maximum required flowrate - 400 kg/hr while producing the stated pressure reduction. It has a 40 to 1 turn down so it can reduce the flowrate down to 40 kg/hr depending on the demand. Nothing wrong with what the vendor is quoting to me. You have to decide whether the pressure reduction to 3-4 kg/cm2 is ok for your system.

Looking at your piping to maintain 60 ft/sec maximum velocity for noise consideration, assuming all Sch. 40 pipe, except 1" Sch 80:

From boiler to PRV - 1 1/2" pipe size is required, assuming saturated steam at 142 psig, 362 deg F, pipe velocity = 54 ft/sec approx., pressure drop = 1.66 psi per 100 feet.

2" pipe downstream of PRV at 3 kg/cm2, 400 kg/hr, velocity = 89 feet/sec (a little high but for short distance may be ok), pressure drop = 1.2 psi/100 feet

3" header at 3 kg/cm2, 400 kg/hr, velocity = 42 ft/sec (ok), pressure drop = 0.18 psi/100 feet

For 1" Sch 80 branches each at 66.66 kg/hr for 6 total branches, 2 kg/cm2, 362 deg F superheated steam, velocity = 92 ft/sec (high), pressure drop = 2.61 psi/100 feet. For 1" pipe Sch. 80 maximum flowrate is 45 kg/hr to maintain velocity at 60 ft/sec approx.

For 1 1/4" pipe Sch 80, 66.66 kg/hr, 2 kg/cm2, 362 F, velocity = 51.5 ft/sec at pressure drop = 0.6 psi per 100 feet. Use 1 1/4" branch lines to users assuming all equal flow of 66.6 kg/hr. Use 1 1/4" branch piping minimum, increase for flows higher than 66.66 kg/hr.

If you relocate the PRV closer to users you could probably run 1 1/2" all the way to where the branch offs are. So if you have say 200 feet of 1 1/2" pipe header at 1.66 psi drop then you would have at total of 3.3 psi drop since pressure drop and velocity is lower for higher pressure section before pressure reduction so you can get away with smaller pipe. Your 3" header is adequately sized though only 0.18 psi/100 ft and 42 ft/sec at 3kg/cm2 so you can keep the PRV where it is just use 3" pipe to users instead of 1 1/2" if you would relocate PRV closer to users.

 

[georgeverghese]

Turndown on Cv for most control valves is about 20:1, you could stretch it to 30:1. Any lower and you'll have to go for a split range pressure control loop.

 

[LittleInch]

A 40: 1 turndown is 400 kg/hr down to 10 kg/hr.

Seems highly unlikely to work well IME.