Two-stage diffusers for propylene control valves

The challenge

A petrochemical plant in Central Asia needed diffusers behind four control valves on its propylene lines. The order came through the manufacturer of the valves. Three duties were involved: two take propylene from 12.28 bar(a) at the valve down to 1.65 and 5.41 bar(a), the third from 22.86 bar(a) down to 8.4 bar(a). The 5.41 bar(a) duty has two valves, each with its own diffuser, and the governing flow cases run up to 825,000 kg/h.

Taking drops that size in one step across the valve trim gives high velocity and noise at the valve outlet. Each diffuser had to share the drop with its valve, connect a 10- or 20-inch inlet to a much larger outlet line, and meet the project’s minimum design temperature of -38 °C.

Our approach & engineering

Axces designed four double-stage diffusers of its 2LDF type, in three sizes, from the process data for each valve: two flow cases per valve, the pressure at the valve and at the diffuser inlet, the operating temperature and the pressure downstream.

Splitting the pressure drop

In every line the valve takes only a small part of the drop and the diffuser’s two perforated stages take the rest. On the line that ends at 1.65 bar(a), the diffuser takes the propylene from up to 11.43 bar(a), a pressure ratio of almost 7. Each diffuser also widens from its inlet to a larger outlet (10 to 18 inch, 10 to 30 inch and 20 to 36 inch), so the expanded gas leaves at a lower velocity. Our article on diffuser design for pressure letdown explains the principle.

In the higher of the two flow cases for each duty:

10" × 18", class 300: 400,000 kg/h at 77 °C, from 20.5 to 8.4 bar(a) across the diffuser
10" × 30", class 150: 230,000 kg/h at 41 °C, from 11.43 to 1.65 bar(a)
2 × 20" × 36", class 150: 825,000 kg/h at 41 °C, from 11.50 to 5.41 bar(a)

Pressure parts for low temperatures

All four diffusers are designed to ASME VIII Division 1 (2017 edition) for -38 to +100 °C, the class 300 unit for 34.2 bar(g) and the class 150 units for 16.7 bar(g), with a 1.5 mm corrosion allowance. Shells and heads are SA-516 Gr. 60, flanges SA-105N and nozzles SA-106 Gr. B, with weld-neck flanges to ASME B16.5 and, for the 30- and 36-inch outlets, ASME B16.47 Series A. Impact testing was included as the design code requires, together with 10% radiographic or ultrasonic examination of the pressure-containing welds and a hydrostatic test of each diffuser.

Two perforated stages that take the larger share of each pressure drop
Outlets two to three sizes larger than the inlets, to bring the velocity down
Designed to ASME VIII Division 1 for -38 to +100 °C, with impact testing
Painted to the project’s painting specification

Scope of supply

1 × two-stage diffuser, 10” × 18”, class 300, Ø600 mm × 800 mm
1 × two-stage diffuser, 10” × 30”, class 150, Ø762 mm × 800 mm
2 × two-stage diffuser, 20” × 36”, class 150, Ø914 mm × 900 mm
Drawings, strength calculations and data sheets for each diffuser
3.1 material certificates, weld procedure qualifications and a manufacturing data book
Closed wooden cases for transport

Results

The four diffusers were painted, packed in closed wooden cases and shipped in July 2020.

Four diffusers in three sizes, for flow cases of up to 825,000 kg/h of propylene
Pressure drops down to 1.65 bar(a) taken in two perforated stages
Designed to ASME VIII Division 1 for -38 to +100 °C
Each diffuser hydrostatically tested, with impact testing for the low design temperature