Industrial Utility Efficiency    

Managing Pressure Regulator Artificial Demand, Part 2

Introduction

This is the second article on how and why pressure regulators can waste air. The first article raised these points:

  • A pressure gauge is like an air use gauge. A higher air pressure results in higher air use.
  • When a machine operates, there will be a minimum regulator outlet pressure during the cycle. This is the minimum “with flow” pressure. Any pressure above this value wastes air by artificial demand.
  • The drop in regulator outlet pressure from the “no flow” value to the minimum “with flow” value is called “droop”. Droop wastes air.
  • Droop is caused by:

  • The regulator valve is like a variable diameter hole. Its size varies with the droop in regulator outlet pressure from “no flow” to “with flow” conditions. As a hole will flow more air with a higher upstream pressure than a lower one so does a regulator valve. So “rated flow” values are often shown based on high supply pressures to exaggerate the regulator performance.
  • The valve is opened by a pilot force. A feedback force opposes the pilot force allowing the valve spring to close the valve.

PSIG Graph

Acuator Movement Graphic

The first 3 causes of droop were explored in the first article. Read the First Part of this Article here. This second article covers regulator design, discusses ways to reduce droop and some special case situations.

The plot of regulator and actuator port pressures shows regulators are not the only cause of droop.  It is also caused by pressure drop in all items in a circuit between the regulator outlet and actuator. This includes, piping, lubricators, Direction Control Valves (DCVs), flow controllers and high loss tube fittings (elbows, Tees).

Tubing from a DCV to an actuator increases the wasted volume (and artificial demand) of a circuit so keep it short. Bigger tubes increase wasted volume but reduce pressure drop which also varies with tube length and the actuator size and speed. So picking the optimal tube size is not easy.

An air pilot regulator with high relief flow acts like a 3/2 DCV. If used at the actuator port it removes other sources of artificial demand. There are valves that combine a regulator and 3/2 function. Some are like a regulator, others a spool valve and some as a mix of both.

Such a valve could be controlled by an existing circuit with a small regulator to set the pilot pressure.

 

Regulator Function

Most regulators are “forward” or “downstream” sensing types. They try to control the downstream pressure. If a “relieving” type they will vent air from the downstream system if the pressure goes above the no flow setting. This is often done by the diaphragm lifting off the top of the valve stem to open a small hole from the diaphragm chamber to the vented spring cap.

Back pressure or pressure relief regulators sense the “backward” or “upstream” pressure and only flow if it is above the set pressure. They are different to a relieving forward regulator. Forward sensing is the most common regulator so this function is not normally stated in the regulator description. As “back pressure”, “differential”, “proportional” and other regulators are different to a forward sensing type, the function is stated in their descriptions.

 

Regulator Design

For a simple regulator, the pilot force opening the regulator valve is created by a mechanical spring. The opposing feedback force is created by the downstream air pressure working on the underside of a diaphragm. There are different pilot, diaphragm, valve and feedback designs. More complex designs cost more.

Common Terms Related to Regulator Designs

Pilot type

Pilot force created by:

  • Spring, mechanical

A mechanical spring.

  • Air, external pilot

Air pressure on top of the diaphragm.

  • Spring with pressure loading

A mechanical spring and air pressure on top of the diaphragm

  • Air, internal pilot.

The regulator has a small spring pilot regulator inside it. The outlet pressure from this regulator works on top of the main regulator diaphragm. Some regulators of this design bleed a small amount of air as part of their operation.

Valve design

 

  • Unbalanced

The valve disc has a pressure difference across it.

This can cause increasing outlet pressure as the supply pressure falls.

  • Balanced

The valve disc has a hole in it which equalises the pressure on both sides. The “non flow” side of the valve disc has a seal system. This can be like a piston with a sealing ring working in a cylinder.  The “piston” and valve sealing edge being the same diameter. As the pressure and the area it works on are the same for both sides of the valve disc, the pressure forces on the valve disc are balanced. A balanced valve minimises supply pressure effects.

Diaphragm chamber design and Feedback source

The feedback pressure on diaphragm should reflect the downstream static pressure. There should be no air speed pressure effects.

  • Open Chamber

High speed air from the valve sealing edge can strike the diaphragm. This causes a higher feedback force. Increased flow causes a lower outlet pressure i.e. more droop.

  • Closed Chamber

A closed chamber stops high speed air hitting the diaphragm and so has less droop.

  • Internal feedback sensing

“Feedback”, “aspirator”, “sense” and “balance” all refer to providing a feedback pressure to the underside of the diaphragm. Internal sensing uses the pressure inside the regulator. For a closed chamber this needs a hole or tube to connect the regulator outlet port to the diaphragm chamber. A hole can suffer air speed pressure effects so a tube with its opening facing downstream is best.

  • External/remote feedback/sensing

The diaphragm chamber is sealed to any other pressure in the regulator. A port connects the external pressure signal to the diaphragm chamber. External feedback allows correction for droop in other pneumatics parts downstream and so aids precise pressure control.

The sketches below are some examples of forward sensing regulator designs. The “air pilot, external sensing” is also a differential pressure regulator. The main design differences for back pressure regulators are:

  • The valve disc is on the diaphragm side (above) of the seal edge.
  • The diaphragm chamber is connected to the regulator inlet not outlet.

Basic Regulator With Labels

Basic design. Spring pilot, relieving. Unbalanced valve, open diaphragm chamber so internal sensing.

Pressure Balanced

Most common design. Spring pilot, relieving. Balanced valve, closed diaphragm chamber. Internal sensing.

AirPilotWithLabels

Air pilot, non-relieving. Balanced valve, closed diaphragm chamber, internal sensing.

AirPilotExtWithLabels

Air pilot, non-relieving, external sensing. Balanced valve, closed diaphragm chamber.

 

How to Reduce Regulator Droop?

Of the four main causes of regulator droop, three are due to the regulator design, size and peak flows. “Design” and “size” need regulator replacement. Peak flows can be lowered by:

  • Setting the regulator to the lowest outlet pressure for the equipment to work properly.
  • For equipment fed by the regulator:
    • Fix air leaks.
    • Fit economiser regulators to actuator return strokes.
    • Use high efficiency nozzles and vacuum venturis.
    • Reduce wasted volume between valves and actuators.
    • Reduce pressure drop between the regulator and the equipment it is supplying.
    • Split the flow and put some of the demand through a second regulator.
    • If there is a high air using device, fit a high relief flow air pilot regulator to it.
    • Using an air tank between the regulator and valve bank.

The fourth cause of regulator droop is from dirty upstream filters (replace them) and undersized piping connected to its inlet and outlet. If upsizing pipes, the new pipe internal diameter should be 3 times the port size of the device supplied by the regulator.

When choosing a new or replacement regulator:

  • Use a regulator with a bigger rated flow as it will have less droop. Beware that too big a regulator may be unstable at low flows and cost more than other options.
  • Use a low spring rate or air pilot design.
  • Use external feedback to correct for downstream pressure drops.

These graphs are for regulators flowing 60 SCFM at an outlet pressure of 76 PSIG.

Same Flow Regulator Comparison

 

Rated Flow,

SCFM

No flow setting

PSIG

Droop

%

Top, small spring pilot

97

90

15.6

Center, big spring pilot

170

78

2.6

Bottom, air pilot

180

76

0

If this regulator is supplying an actuator:

  • The bigger regulator will use 13% less air.
  • The air pilot regulator will use 15.6 % less air.

 

Special Cases

Some special cases need extra thought when choosing a regulator. These include:

  • Economiser regulators:
    • Are used between a valve and an actuator to reduce the pressure and air used during one of the actuators strokes. For example where the actuator needs to create more force in one direction than the other, it would be used on the lower force direction.
    • Must allow reverse air flow. A regulator with
      • A balanced valve doesn’t allow reverse flow. A check valve (often built in) with this regulator will allow reverse flow. At the start of reverse flow air passing through the check valve lowers the outlet pressure. The regulator valve then opens allowing easy reverse flow.
      • An unbalanced valve opens naturally at the start of reverse flow due to supply pressure effects on the feedback force. Supply pressure effects can work to reduce droop at end of stroke. So an unbalanced regulator may be better than a balanced one and will likely be cheaper.
      • Add to the wasted volume (and artificial demand) of the piping between the valve and the actuator. So using a bigger regulator may not return the expected savings.
  • Outlet set pressure close to supply pressure.

While air pilot regulators give very low droop, the pilot pressure must be sufficient to overcome the force of the valve spring.

This can require:

  • A minimum pressure difference between the supply and outlet pressures.
  • A higher system pressure that increases the air and power use by the wider air system.

Options include using:

  • An external air pilot regulator where the supply to the pilot regulator is kept above the supply pressure when the main regulator.
  • A pressure loaded spring regulator. Best with an external feedback regulator to supply the loading pressure.
  • An oversized spring pilot regulator and reduce the peak flow.

 

Conclusion

This article has discussed different aspects of regulator design and how they affect air wasted by droop. Some ways to reduce droop have be shown and some special case situations discussed.

By taking care with regulator selection and installation, regulators can save large amounts of air instead of wasting it.

 

Read the First Part of this Article here.

 

For more information please contact Murray Nottle, The Carnot Group. mnottle@carnot.com.au, www.carnot.com.au.

To read similar Instrumentation Technology articles visit http://www.airbestpractices.com/technology/instrumentation.