PSV outlet piping to atmosphere design pressure?

[ropumar]

Hi,

I believe that outlet pipe should have design pressure same as the boiler design pressure and have been defending that at my work.

However, people at my work argument that since outlet pipe discharge to atmosphere, piping design pressure can be low(5K Class), since pressure over all the length of outlet will be {atmosphere + the pressure loss trough pipe}. I believe this is wrong, pressure at pipe will be closer to operation of boiler when PSV discharge happens. If I am wrong, please enlighten me.

What is the right answer? Can a 5K CLASS pipe be used for discharge of a 16barg Boiler as picture shows?

 

[Latexman]

Read the applicable Code for your jurisdiction. There is usually a mandatory limit on backpressure, so while the pressure class should exceed this, it should not do so EXCESSIVELY (i.e. expensively).

What Code applies?

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[GHartmann]

The outlet pipe rating does NOT have to be the same as the inlet pipe. In fact the PSV itself will not be provided in that manner. It will be provided with the lowest pressure class.

I am not sure about the EU / EC system but the for the ANSI / ASME system that will be 150#.

The whole question is rather moot as the only flange in the system will be at the PSV outlet. The rest will be pipe which is the same for nearly every pressure rating except for really high pressures (1500# / 2500#)

 

[EmmanuelTop]

Both Latexman and GHartmann are correct, and the OP question contains incomplete data. There is no reason to go beyond certain margin above the maximum calculated backpressure, as far as we are talking about the piping class.

Much greater concern is which type of relief device to use. There is no sufficient data to perform pressure drop calculation but if you end up with backpressure in the range of several bars, conventional PSV will not work no matter which piping class you use.

Dejan IVANOVIC
Process Engineer, MSChE

 

[don1980]

I think this is a homework problem. It has value as an academic exercise, but it's not realistic. That is, it's very hard to imagine a case in which one would need to run 100m of piping in order to vent steam to the atm. If there were such a case, it's unlikely that this installation would be approved by a boiler inspector due to the length of that tailpipe.

In steam boiler applications like ASME Sec I, there are no inlet and outlet pressure drop rules. Instead, the requirement is that there must be a minimal amount of inlet and outlet piping.

In this specific hypothetical case (with the long tailpipe) the pressure in that tailpipe is a function of the pressure drop caused by the 20 tons/hr of flow. The pipe class is selected based on the calculated pressure. With very few exceptions, 150 class piping is appropriate for atm discharges.

 

[LittleInch]

I don't know what class 5K, 10K etc is - I've never seen pipe design defined this way before. 5,000 what?

Anyway, ropumar, you are not correct. What you need to look at is how each element in the system works and what are your fixed points.

The fixed items are atmospheric pressure and our boiler pressure and steam flow. As the pressure exceeds your limit, PSVs can operate in a variety of ways, either "pop off" where they open fully or variable where they gradually open fully over a set pressure range. either way, at full flow, they are sized to allow only a set amount of flow through their orifice / opening otherwise the piping and other systems downstream would need to be huge. These orifices create a significant pressure drop

In terms of your piping, you start at the far end at atmospheric then add the back pressure. This could be considerable I the case of steam as it will expand from one end to the other relating in higher and higher velocities, however there are simulations and programs to calculate this. I agree with don, 100m of 6" pipe sounds like a long way in a small pipe and seems really quite odd.

There are valves which can ignore the downstream back pressure, but you do need to specify what this is to your PSV vendor so he can calculate the orifice size to be able to relieve your max flow.

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

 

[Latexman]

It could be a process boiler and like ASME Sect. VIII. 100 m of tailpipe is still hard to explain, unless it is indoors, and that in itself is hard to explain too.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[ropumar]

The example I gave was hypothetical.

I think you guys clarified now, the pressure at pipe will be only the pressure drop of outlet pipe to discharge and it seems I was wrong.
5K would be 5barg or 75psi, which I think would suffice considering what you guys answered.

Thank you very much.

When calculating velocity on said pipe, will I consider STEAM at which pressure for calculation? Because what troubles me, is that if I consider Steam at the low pressure equal to the maximum pressure drop, the volumetric flow will be high and thus velocity will be very high, above 200m/s, which is higher than the exhaust steam recommended velocity tables I have.

 

[ropumar]

The instalation is in reality about 27m from boiler to funnel top (pipe outlet).

Outlet diameter of PSV and outlet pipe, is not defined yet.
The 6inch for steam at ATM considering blocked discharge and requiring the total flow trough PSV, would create too high velocity.

Is the consideration of blocked discharge and 20ton/h of mass flow unrealistic for a boiler design criteria? (the boiler total capacity is 20ton/h)

Or is the design criteria correct and pipe diameter should be increased to about 12inches and to achieve velocity below 90m/s?

Sorry for the perhaps trivial questions, I am starting on the job and don't have much experience.

Boiler is used for heating of FPSO cargo oil tanks.

 

[LittleInch]

I think that's what many of us were thinking, i.e. 6" just looked too small if this was designed for the entire boiler flow.

A blocked outlet cause is common and you need to protect your vessel against any issue, whether likely or not. The consequences of a boiler explosion are just too high to think about so if you need a bigger valve and pipe, install a bigger valve and pipe. It should be sized for max flow. Only you can determine what that max flow is, but it sounds like 20 tons/hr to me. Clearly this should only be for a short time before the boiler shuts down, but you can't allow a pressure rise to occur above MAWP.

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

 

[Latexman]

When calculating velocity on said pipe, will I consider STEAM at which pressure for calculation? Because what troubles me, is that if I consider Steam at the low pressure equal to the maximum pressure drop, the volumetric flow will be high and thus velocity will be very high, above 200m/s, which is higher than the exhaust steam recommended velocity tables I have.

Steam is a compressible vapor, so you will need the pressure at the point where the velocity is calculated/applicable. Velocity will be highest at the tailpipe exit. It could approach the speed of sound, thus indicating choked flow at the tailpipe exit. That is a real physical upper limit to the velocity at the tailpipe exit.

The PSV is there as the last layer of protection in case something goes wrong. As such, the PSV should never/rarely activate, and the tailpipe will never/rarely see flow. Some companies limit the tailpipe design below Mach 1. I think Mach 0.7 is common. Your 200 m/s is close to that. Some companies will allow Mach 1 for such a rare event. Check what is your company's policy.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[itdepends]

In the OP's design- I would expect to see a drain in the low point immediately after the PSV to allow any condensate that forms due to a passing PSV to drain- otherwise you could conceivably fill that 100m discharge leg up over time which would alter the PSV lift pressure and provide a bit of a shower when it did lift.

As a chem eng/metallurgist the first part of any answer I give starts with "It Depends"

 

[Drexl]

Calculate the pressure drop in the outlet pipe and there you have your maximum pressure. Select design pressure accordingly.

 

[ropumar]

Thank you all, you guys were very helpful.

 

[georgeverghese]

It is not advisable to install the PSV below the point where it discharges into the vent. The PSV tail pipe should always slope DOWN to the exit - this is particularly important when the vent stream can condense in the tail pipe. Low point drains provided in exit piping can plug up due to corrosion.

Sizing of the discharge line should be based on compressible gas flow for steam, with the permissible backpressure at the RV exit appropriate for the type of PSV selected and the SP of the RV. As suggested, keep velocity at less than Mach 0.7.

Inlet pressure loss to the RV should not exceed 3% of SP. In some companies, it is customary to do an AIV (acoustic induced vibration) coarse screening exercise also to see if there is a need to increase exit pipe schedule.