Question

5. The flow characteristics of the pressure regulator having a setpoint pressure of 25 psig when the input pressure is 100 psig is shown in Figure 1. This regulator maintains pressure for a system designed to supply air to equipment at a flow rate of 4 SCFM during average system demand and up 6 SCFM if all the equipment is operating at the same time. The manufacture of the regulator will report flow rates at 10% and 20% droop on his specification sheet based on the setpoint pressure since he doesn't know how it will be used for a particular application. What flow rates will the manufacturer report on the specification sheet below for these droop conditions? a. Capacity Outlet Pressure (psig) Inlet Pressure (psig) 10% Droop 20% Droop SCFM SCFM At what pressures do 10% droop and 20% droop occur? c. What is the pressure downstream of the regulator: i.) When there is no demand ii.) During average system demand ii.) During maximum system demand and Indicate on Figure 1 the point at which the regulator will be operating at during average system demand, maximum system demand, 10% droop, and 20% droop. d. e. The shop manager wants to add several new pieces of pneumatically operated equipment that will require an additional 6 SCFM. How will the system respond when all of the equipment is operating and what would you recommend to prevent this situation? Outlet Pressure (psig) 3

Answer 1

a.

(10% droop) - Outlet pressure = 22.5 psig ---------- Capacity = 7.4 SCFM

(20% droop) - Outlet pressure = 20 psig ----------- Capacity = 10.3 SCFM

b.

Outlet Pressure at 10% droop = set pressure - 0.1* set pressure = 22.5 psig

Outlet Presssure at 20% droop = set pressure - 0.2* set pressure = 20 psig

c.

i) When there is no demand flow rate is 0, the downstream/ outlet pressure = set point pressure ( from graph) = 25 psig

ii) During average system demand flow rate is 4 SCFM (given), from graph, pressure = 24 psig (red line)

iii) During maximum system demand flow rate is 6 SCFM (given), from graph, pressure = 23.5 psig (yellow line)

e.

If an additional 6 SCFM is added to maximum demand, the total demand will be = 6 + 6 = 12 SCFM

The outlet pressure from the graph corresponds to 17.5 psig (purple line) < set point pressure

Figure 1 25 PSIG Air Regulator Flow Characteristics 30 28 26 h 20 18 16 14 12 0 1 2 3 4 5 6 78 9 10 11 12 13 14 Flow Rate (SCFM) Page 8 of 8

Some steps which can be taken to reduce droop are;

• Choosing a regulator such that inlet pressure closely matches actual system pressure.
• To increase the diaphragm area, decrease the spring rate and/or decrease the length of the valve stroke.
• Eliminating the load spring and use a dome loaded/ air loaded or pilot driven regulator, due to enhanced sensitivity to demand.
• Specify a different regulator whose flow requirement is not in choked region (12 SCFM in this case)

c.

Average system demand - Red line

Maximum demand - Yellow line

10% droop - Orange line

20% droop - Blue line

Figure 1 25 PSIG Air Regulator Flow Characteristics 30 28 26 h 20 18 16 14 12 0 1 2 3 4 5 6 78 9 10 11 12 13 14 Flow Rate (SCFM) Page 8 of 8