Boiler Safety Valve Discharge piping 1

[farishta]

Dear all,

Please see attachment for my query related with discharge pipe routing of PSV on a 70BarG Boiler Drum.

There are two PSV's on the drum and outlet piping has to be connected to a Silensor to meet the environmental sound limits.

As there is a limitation (approved by law for this project) on the maximum height of the structure, the PSV discharge piping has to go below the PSV nozzle so that it can be connected to the silensor inlet.

My Queries are

1) Is it prohibited to do a discharge pipe routing like this (see attachment

http://files.engineering.com/getfil...e5e-48dd-9a68-6769cfb89821&file=Untitled.png)

for a HP PSV discharge piping provided the discharge pipe routing satisfies the max. back pressure of 10% ?
2) Is there any safety concerns for this type of pipe routing ?

Appreciate your expert insight in this

Thanks

 

[saplanti]

You should check this with local authority, but I strongly believe that the silencer height/location should not be limited by the building height. You should be able to fit/support the silencer above the roof from the roof structure. So you would not have the problem with pipe routing. Additionally I would bring the silencer as clase as possible to the above the drum. This will reduce the pipe lateral displacement under thermal load.

I would not recommend the sketched arrangement at all, this will cause problem even though you may provide drain for condensation on the lower section of the piping. More elbow numbers will increase the losses, this will increase the backpressure of the drum safety valve. This may require sometimes special large safety valves to meet the backpressure requirements.

 

[EmmanuelTop]

PSV relief and the subsequent noise do not fall under "normal operating conditions", hence the noise mitigation requirements likely do not apply for the relief scenario. This can be argued with the authorities, or whoever believes that silencer is required at the PSV discharge. If you would be continuously discharging through that pipe, then yes, something would have to be done with the noise, but if it is under relief/emergency conditions only it is hard to justify stringent noise requirements.

The piping layout is unacceptable. It creates dead leg where liquid will certainly accumulate and impede relief capacity/capability. As pointed above, vibrations will also be significant.

As a general rule, silencers and other devices that could potentially get plugged and block or reduce relief flow, should not be installed in the PSV discharge pipework.

Dejan IVANOVIC
Process Engineer, MSChE

 

[georgeverghese]

If you have no choice but to arrange the discharge piping as shown, a liquid drip leg with automated drain and LSHH should be fitted on the low section of the piping.
Believe this practice of using silencers on boiler PSVs' in the steam industry is common. A bypass backup bursting disc around the silencer would be a good idea, in case the silencer is corroded. Would suggest some CRA material for this silencer also.

 

[MJCronin]

You are suggest a dangerous routing, and is unlike all manufacturer approved routings and could hurt people.

Your routing could develop dangerous slugs of water that could tear the system apart and kill people

You need to be ready to testify in court how you were reluctant to challenge the maximum height restriction and how your MBA boss pressured you into making this bad decision.

"saplanti" gives good advice ...

MJCronin
Sr. Process Engineer

 

[don1980]

farishta - Based on your profile info, you're located in The Netherlands so I assume this is for a European facility. If so, you need to get a copy of the ISO standard that applies to this type of steam boiler. I don't work with steam boiler very often so I don't remember the specific differences between the requirements in that ISO standard and ASME Sec I. I recall that they are similar, but there are some differences.

If this boiler was in an ASME jurisdiction, then you wouldn't be allowed to install a muffler (ref: ASME Sec I, PG-71.4). Be aware that the rules for steam boilers are different from those for pressure vessels - that's true in EU countries and in ASME jurisdictions. For example, the pressure drop rules don't apply to steam boiler applications. Instead, the ASME Sec I rules prescriptively state that there must be as little as possible piping on the inlet and outlet side. ASME Sec I allows a muffler on boilers that operate at low T & P, but not for "high-temperature water boilers" which operate at a pressure greater than 160 psi or a temperature over 250F.

Boilers in EU countries and those in ASME jurisdictions must be inspected and approved by an independent boiler inspector. Even without the muffler, I don't think any boiler inspector would approve a line pocket like the one shown in this installation. That pocket creates the dangerous possibility of the line being plugged in the wintertime due to ice, and/or the possibility of a "steam explosion" caused by high temperature steam mixing with water trapped in the pocket.

 

[farishta]

saplanti,EmmanuelTop,georgeverghese,MJCronin,don1980 Thank you all for the detailed clarifications and recommendations.

I knew from the beginning that this discharge piping arrangement is not the recommended way and to reaffirm my thoughts i presented it before you guys and BTW the sketch i presented was only one of the last resort options.

As don1980 said this is for EU.
Once again thank you guys

Regards
Farishta

 

[saplanti]

I suggest, before going further detail, you should confirm the selected safety valve with considered backpressure will work. Consider the pressure loss on the silencer (get it from the vendor, or , I suggest, you nominate the back pressure on the silencer side) in your calculation. Lower silencer loss will result larger size silencer.

If you do not confirm with calculation, you will be upset in the first operation, it does not matter if the project is in the EU or anywhere else in the World.

 

[farishta]

Thanks saplanti, i will definitely go ahead as per you advice.