One steam vent silencer for two safety valves at a UK refinery

The challenge

A refinery in the United Kingdom needed one silencer for the steam relieved by two safety valves. Each valve discharges 60,820 kg/h from 45 bar(g) and 259 °C at its inlet, so the silencer has to take up to 121,640 kg/h of steam. The discharge piping brings that steam to the silencer at up to 4.5 bar(g) and 175 °C, and the silencer releases it to atmosphere.

A safety valve relief is one of the loudest events on a refinery, and it happens when the plant is already in an upset. The silencer had to bring the relief down to a guaranteed level at 1 m, carry the pressure in its inlet sections as pressure equipment under the PED, and cope with the discharge piping moving as it heats up.

Our approach & engineering

Axces designed a single silencer of Ø2,000 mm and 2,900 mm overall length with two inlets on its dished head, one for each valve.

Diffuser and absorption

The steam enters through diffusers that break the incoming jet into many small jets. It then passes a perforated stainless steel bundle, with mineral wool held behind glass cloth and stainless steel gauze, before it leaves the silencer. Axces guarantees 115 dB(A) at 1 m, 90°, in free field, which is a sound power level of 126 dB(A). Our article on diffuser design for pressure letdown explains the principle.

Sliding inlets for moving pipework

The discharge lines grow as they heat up. Instead of passing those loads into the silencer, both inlets slide in stuffing boxes that take -10 to +50 mm of axial movement and ±2.5 mm laterally.

Pressure parts and construction

The inlet sections are designed for 5 bar(g) and 259 °C, with a minimum design metal temperature of -7 °C and a 3 mm corrosion allowance, to ASME VIII Division 1, and fall under the PED as Category I, conformity assessment module A. The shell itself is atmospheric. The shell and heads are SA-516 Gr. 60, the nozzles and diffusers SA-106 Gr. B, the flanges SA-105N and the perforated bundle and drain stainless steel 304. The inlets and first diffuser received 10% radiographic or ultrasonic examination, the supports 100% magnetic particle or dye penetrant testing, and the design includes a wind load calculation.

• One silencer for two safety valves, with an inlet for each
• Diffusers and a stainless steel absorptive bundle
• Sliding stuffing-box inlets for +50 mm of pipe movement
• Inlet sections to ASME VIII Division 1, PED Category I
• Blasted to Sa 2½ and shop-primed with 75 µm zinc silicate

Scope of supply

• 1 × steam vent silencer, Ø2,000 mm × 2,900 mm, for two safety valves of 60,820 kg/h each
• 2 × 8” class 150 weld-neck sliding inlets with stuffing boxes, and a 2” drain
• Wind load calculation
• Surface preparation to Sa 2½ and zinc silicate shop primer
• EU Declaration of Conformity (PED Category I, module A)

Results

The silencer was fabricated and primed at Axces Production in Poland and released for shipment after the client’s own inspection.

• Guaranteed 115 dB(A) at 1 m, 90°, free field
• Up to 121,640 kg/h of relieved steam through one silencer
• Sliding inlets that keep pipe loads off the silencer
• Inlet sections declared under the PED as Category I pressure equipment

Related reading

• Steam vent silencers: sizing for safety valves, start-up vents and blow-off: the data a safety valve silencer is sized from
• Diffuser design for safe pressure letdown and venting: how a diffuser breaks one choked jet into many quieter ones
• Pressure vessel design and certification: PED 2014/68/EU essentials: the categories and conformity modules behind the inlet sections
• Understanding dB(A) reduction targets: how to specify acoustic performance