I've never seen silencers installed on steam safety discharges. I'd bet you'd have to jump through some pretty impressive calculations and code hoops to get that approved.
Silencer most of time instal for avoid noise and vibration in regular. In our plant use for this purpose. But there is some problem I met is one of the silencer in our plant don't work. Does any one can suggest the solution to solve this problem? Does it cause by un-properly piping or other reason?
Thanks to Every one can help.
The silencer is only part of the design. You could be having noise generated in the piping to the silencer or within the letdown valve itself. Alternatively, the silencer could be undersized for your operating conditions.
You are all missing some important issues.... backpressure and safety of the vent pipe installation.
The design code to review is ASME B31.1. Within this Code is a manatory Appendix II which discusses some of the issues of safety/relief valve design, discharge conditions and stress levels in the adjoining header piping.
Look at ASME B31.1 -Paragraph II-5.8.
It states that silencers must:
"...be properly sized to avoid excessive backpressure" and
"...vent stacks shall be properly supported to avoid excessive loading on the valve discharge flange"
Please note that if the vent system is improperly sized ( too small) and if too much backpressure exists, what can happen is called "blowback". This is where steam escapes locally (at the SRV) and can cause serious injury and death.
TBP, many silencers are now being insatlled on new HRSGs and new steam system on all of the new combined cycle plants across the country. Look at some of the pictures of the installations by VOGT, DELTAK and Nooter-Erickson.
When you design silencers there are two aspects you have to look at.One to have lower back pressure to avoid any effect on SRV performance. Other is to have enough back pressure to get the optimum performance from the silencer. Silencers is designed for a specific flow condition with a certain back pressure to reduce the noise level of the SRV. Usually stem silencers for SRVs designed with reactive (diffusser) and absorptive sections. Both diffiusser and size of the flow path of the absorptive section and length of it is decided to achieve required dB reduction and pressure. Most of the SRV manufacturers will be able to tell you the maximum back pressure of the the down stream piping can create before it effect the SRV. Most of the standard silencers have 20 -25 kPa pressure drop. But sometimes SRV manufacturers ask to increase the pressure drop through silencers. In these cases we include an orifice plate between mating flanges of the silencer and SRV
Thank you for replying. MJCronin approach was exactly what I was looking for. I contacted several silencer companies including Universal - which did not replied me. I would like to know if ASME states a maximum back pressure for silencers as "X"% of the valve upstream pressure.
The valves I am talking about are safety relief valves, 50 t/h, 10,2 kg/cm2 pressure. They produce very high noise levels when they are open and we need to silence them.
Some engineers said ASME do not allows the use of any silencer with relief valves. Other said ASME allows 10% of the upstream pressure as backpressure. That is in our case the maximum backpressure shall be 1,2 kg/cm2 in the silencer + piping after de valve.
Do you know if ASME code has some specific approach for silencers used with safety relief valves? I will look to ASME B31.1 Appendix II.
I don't think that there is an issue with using silencers on relief valve discharge pipng. As MJCronin pointed out they are used on new CHP plants. We are installing them on a CHP scheme on a number of relief valves. One thing to note though is that the relief valve should be sized for the relief valve actual capacity rating (un-derated) and not on the relieving capacity required. The manufacturers will give the associated pressure drop for that flow and so long as that pressure, and also the piping pressure loss, is within 10% of set pressure the relief system should conform to the ASME standard. If the back-pressure is too great then you may have to consider using a balanced bellows relief valve.
I used to work sizing PSV and silencers for different applications. This info can give and idea of what to expect, first of all as our colleges have said, you must count for the back presure that your vent piping system has, if this back pressure is too high a bellow type valve should be installed, however, be careful with backflow taht can ocurr with high backpressures. To avoid tis the vent piping and the silencer have to be sized big enough to reduced this effect, you can find a good description of this in the consolidated's catalog for PSV, also there is an artcle by Max W Bnejamin from Dresser Valve division dealing with the design of vent piping. Now, you have to consider the reaction forces generated when the valve opens and add the effect of the silencer to it. Again consolidated has a chart where you can find a good approximation of the value of this reaction force. (if you want I could scan this info and send it to you).
Other thing is the noise level generated by the valve, that you can measure with and instrument or theretically by using a log/log graph where the mass flow rate vs sound level at a set pressure.
Where is this valve installed? the farther the valve to the personnel the smaller the silencer. You have a value which is the noise level at the discharge of the valve (theoretical or by mesuring with and instrument), the acceptable noise level to the point where personnel will be located is giving by OSHA (90 dBA for 8 hours exposure), now you can use the following ecuation:
SPL= (PWL-20 log r) + 2.5
Where
SPL = sound pressure level
PWL = sound powel level at the dicharge of the valve
r = distance in feet from PWL source (the PSV)
If you have let's say a valve with a PWL = 100 dB and your satff will be working at 500.23' from the valve, the equation will give us
SPL = 100dB - (20*log 500.23) + 2.5 = 48.52dB
which means that we will have a lower SPL than the rewuire by OSHA, therefore we wouldn't need to use a silencer.
now if ont he contrary our PSL where let's say 120dB, we will need to reduced 120dB-90dB= 30dB with a silencer. This can be a bit more complex if our crew is working in different areas with different noise levels, this effect must de added to find out what we need to install. Again OSHA has more information regarding this topic.
Me again,
I have just noticed a mistake from my reply.
What I said,
'One thing to note though is that the relief valve should be sized for the relief valve actual capacity rating (un-derated) and not on the relieving capacity required.'
and what I meant to say was,
'One thing to note though is that the SILENCER should be sized for the relief valve actual capacity rating (un-derated) and not on the relieving capacity required.'
My apologies and well done to those who spotted it. Tame excuse but I have been spending the bank holiday writing O+Ms!!
