A high-pressure steam vent silencer for a Scandinavian refinery

The challenge

A refinery in Scandinavia needed to silence a high-pressure steam vent. At the sizing case the vent releases 24.7 t/h of superheated steam at 33 bar(a) and 420 °C straight to atmosphere. A pressure ratio of about 33 to 1 is far above the critical ratio, so an unsilenced vent would discharge as a choked, underexpanded jet, one of the loudest noise sources on a process site.

The silencer had to bring that vent down to 85 dB(A) at 1 m and 90°, in free field. Because the inlet section holds the full steam pressure, it is pressure equipment in its own right and had to be designed to ASME VIII Division 1 and certified under the Pressure Equipment Directive. On top of that came the refinery’s own requirements for external coating, wind load and lifting.

Our approach & engineering

Axces sized the silencer from the venting case against a validated aero-acoustic calculation, then split it into the two parts that do different jobs: a pressure-bearing diffuser that takes the letdown in steps, and an atmospheric absorptive section that removes the noise that remains.

Five-stage pressure letdown

Instead of one violent expansion, the steam passes through five diffuser stages, each larger than the last and each designed for the pressure it sees. Every stage replaces one large jet with many small jets at a moderate pressure ratio. Small jets generate far less sound energy and push what remains to higher frequencies, where the absorptive section works best. Our article on diffuser design for pressure letdown explains the principle.

Absorptive section for hot steam

The 1,100 mm shell is 2,600 mm long and made of SA-516 Gr. 60 carbon steel. Inside, perforated stainless steel internals (304 and 316) hold mineral wool fill behind glass cloth and stainless steel gauze, so the fill cannot migrate or erode under repeated steam venting. A weather hood keeps rain out of the outlet, a drip ring sheds water off the shell, and a thermally sprayed aluminium (TSA) coating protects the outside.

Pressure equipment and documentation

The inlet section was designed to ASME VIII Division 1 and certified under PED 2014/68/EU as Category II, conformity assessment module A2, with a notified body. Materials carry EN 10204 3.1 certificates, welding procedures are qualified to EN ISO 15614-1 or ASME IX, and the inlet section received 10% radiographic or ultrasonic examination.

• Five diffuser stages, each designed for its own pressure
• Absorptive section with stainless steel internals and retained mineral wool
• Inlet section to ASME VIII Division 1, PED Category II, module A2
• Wind load calculation to the refinery’s specification and lifting lugs calculated to the specified lifting standard
• TSA external coating to the refinery’s coating specification

Scope of supply

• 1 × steam vent silencer, Ø1,100 mm × 2,600 mm, for 24.7 t/h of steam at 33 bar(a) and 420 °C
• 4” class 300 inlet and 2” class 150 drain, ASME B16.5 weld-neck flanges
• Weather hood and drip ring
• TSA external coating
• Wind load calculation and lifting lug design calculation
• Technical handover documentation, including the EU Declaration of Conformity (PED module A2) and 3.1 material certificates
• Installed mass 710 kg, plus 130 kg for the weather hood and 35 kg for the drip ring

Results

The refinery received a steam vent silencer guaranteed to bring a 33 bar(a) superheated steam vent down to 85 dB(A) at 1 m, with the pressure-bearing part certified as pressure equipment and the full documentation package behind it.

• Guaranteed 85 dB(A) at 1 m, 90°, free field
• 24.7 t/h of steam at 33 bar(a) and 420 °C let down in five stages
• Inlet section to ASME VIII Division 1, certified to PED Category II, module A2
• Complete technical handover documentation, including the declaration of conformity and material certificates

Related reading

• Steam vent silencers: sizing for safety valves, start-up vents and blow-off: the data a steam vent silencer is sized from, and the noise target it must meet
• Diffuser design for safe pressure letdown and venting: how a multi-stage diffuser breaks one choked jet into many quieter ones
• Sizing a vent silencer for high-pressure gas blowdown: the same principles applied to gas venting and depressuring
• Pressure vessel design and certification: PED 2014/68/EU essentials: the categories and conformity modules behind the inlet section’s certification