Industrial Utility Efficiency    

Managing Pressure Regulator Artificial Demand, Part 1

Pressure regulators are everywhere compressed air is used. These simple devices, essential for safe and steady equipment operation, can be a big waster of compressed air. This article shows how with proper regulator selection, installation and setting management you can save compressed air and lower system pressures. This article looks at regulators on production equipment not central regulators or Process Flow Controllers.

 

Artificial Demand

Ussage WasteWhen walking around your factory, look at what the regulator pressure gauges are doing. As equipment downstream of a regulator cycles, is the gauge steady, moving quickly, or moving up and down slowly? Video fast-moving gauges then watch in slow motion.

Think of a regulator pressure gauge as a compressed air use meter. The higher the pressure the more air is being used. The more the needle moves, the more air is being wasted as artificial demand.

The amount of compressed air used by actuators, including air motors and diaphragm pumps, varies with gauge pressure. For nozzles (holes, vacuum venturis, leaks) compressed air use varies with absolute pressure (gauge + 14.7 psi).

The table below shows relative compressed air use at different pressures.

PSIG

Actuator

Nozzle

100

100 %

100 %

90

90 %

91.3 %

85

85 %

87  %

80

80 %

82.5 %

 

For most actuators, the actuator port and regulator outlet pressures that matter most are while the actuator is moving, not when it is stopped. This is the lowest pressure value shown on a gauge during the cycle. Air is wasted when the pressure rises after the actuator stops.

Actuator pressures graphic

Gauge Graphic

Regulator Droop Observations

Droop:

  • Is how much the regulator outlet pressure changes with flow. A regulator outlet pressure set at 100 psig with “no flow” that drops to 85 psig “with flow” has a “droop” of 15 psi. It “droops” from 100 to 85 psig.
  • Is due to how the regulator valve is piloted, where pressure feedback is sensed (internally or externally), the regulator valve design (balanced or unbalanced) and internal pressure drop.
  • Wastes air
  • Forces the air compressors to operate at a higher pressure. The minimum system pressure is the “with flow” regulator outlet pressure plus droop. Less droop allows a lower system pressure.

 

Understanding How Regulators Work

The regulator valve is a “hole” whose size changes with a pressure difference. The pressure difference is the “no flow” outlet pressure setting minus the “with flow” outlet pressure i.e. the regulator droop.

As the upstream pressure rises and falls, so the compressed air flow through the “hole” will rise and fall. As a hole is a nozzle, air flow varies with absolute pressure. A hole with 100 psig upstream will flow 28% less air that if supplied with 150 psig.

How quickly the “hole” size changes with pressure difference, depends upon the “spring rate” of the pilot force opening the valve. Opposing the pilot force (to keep the valve closed) is the feedback force. The feedback force is usually the outlet pressure acting on the regulator diaphragm.

Spring rate means a spring compressed to say 2” will have less force if only compressed to 2 ¼ ”. If this same spring creates the pilot force on a regulator valve, for the valve to open ¼” (spring extends from 2” to 2 ¼”) the feedback force (downstream pressure) needs to be less than if the valve was closed. As flow through the regulator starts and stops, the valve needs to open and close. To allow this, the downstream pressure must go up and down.

Most general regulators use coil springs so their droop is mainly due to spring rate. Air pilot regulators have no spring and so no droop due to spring rate. This is why they’re used for precision regulators.

A regulator valve can only open so far. With a fully open valve, the regulator can’t control the downstream pressure and has “lost regulation”.

Did you know that if you measure the regulator “no flow” pressure setting and “with flow” inlet and outlet pressure, you can use the regulator catalogue curves to estimate how much air is flowing i.e. as a flow meter. Not so good for air pilot regulators due to the very low pilot spring rate.

Did you know that if you measure the regulator “no flow” pressure setting and “with flow” inlet and outlet pressure, you can use the regulator catalogue curves to estimate how much air is flowing i.e. as a flow meter. Not so good for air pilot regulators due to the very low pilot spring rate.

 

What affects a regulators ability to keep a steady outlet pressure?

How well a regulator can keep a steady outlet pressure depends upon many factors.

You’ve already read about spring rate effects and that other parts of a regulators’ design affects how it operates. These will be discussed further in a later article. Supply pressure effects and the original regulator sizing are discussed here.

 

Supply Pressure

Remember that a regulator is part of and affected by the wider compressed air system and that a regulator valve is like a “hole”. If the supply pressure at the regulator drops, less air will flow through the “hole” so its’ size needs to increase. For the hole size to increase (valve open further), the outlet pressure needs to droop further below the no flow setting. So drops in supply pressure increase droop from spring rate effects. Large drops in supply pressure can cause “lost regulation”.

Pneumatic components have much higher pipe air speeds at rated flows than are acceptable in air mains. Based on 43 regulators of different design, manufacturer and size, port air speeds are between 200 and 300 ft/sec (60 - 90 m/sec) compared to the < 20 ft/sec (<6 m/sec) considered as best practices for air mains. Low air speeds result in low pressure drop, efficient system operation and stable regulator supply pressure.

Using plastic tube of the same nominal bore as the regulator port (¼” bore tube with ¼ NPT port), will result in unsteady supply pressure to the regulator. This in turn will increase droop and may result in lost regulation, all of which will result in air waste and affect equipment operation.

As a guide the pipe bore, either side of a regulator, should be 3 to 4 times the port size of the regulator. This is except where the regulator is between an actuator and its control valve where small tube should be used. If the regulator is part of a service unit with a filter, the filter element pressure drop lowers the supply pressure drop to the regulator.

So before blaming and changing a regulator for poor pressure control, change the filter element of its service unit and check the pipe sizes either side of it. Otherwise a different regulator may do no better.

 

Sizing

For easier regulator selection, manufacturers provide “rated” flows and flow curves - but at what “rating” conditions?

• Most spring-piloted regulators are based on the outlet pressure drooping from 90 psig to 75 psig. But:

  • Some manufacturers test flow using a 100 psig supply.
  • Others use a 145 psig supply.
  • Remember a hole supplied with 145 psig air will flow around 30 % air more than if supplied with 100 psig.
  • So a smaller (cheaper) regulator tested on a 145 psig supply could have the same rated flow as a bigger (more expensive) regulator tested on a 100 psig supply.
  • Which one do you think is more likely to be bought ? The smaller, cheaper one.
  • Which one will have more droop, is more likely to “loose regulation” and will waste more air ? The smaller, cheaper one.

If you select a spring pilot regulator near its “rated flow” then, you are designing your equipment to waste compressed air, have operational problems and need high compressed air system pressures.

• Air pilot regulators (low spring rate) rated flows are with a fully open valve. However as with the spring piloted regulators some are tested with a 145 psig supply while others a 100 psig.

You can see how easy it is to select an undersized regulator by not considering the test supply pressure compared to the supply the regulator will use. Before rushing out to replace a regulator you know is undersized, make sure you know what, balanced valve, internal and external feedback and aspirator tube relate to and how they affect regulator selection. These terms will be covered in the following article.

 

Are Your Regulators Properly Set?

Often over time, the regulator no-flow settings increase and sometimes regulation is lost. The regulator doesn’t drift but it is machine operators trying to keep their equipment working reliably.

The pressure drop of air compressor room filters increases, as they clean the air, resulting in the system pressure falling. Droop increases and low pressure affects production equipment operation. To immediately fix the problem, the operator increases the regulator setting pressure but the “with-flow” setting is a guess or a “memory”.

Eventually air compressor settings are increased and filter elements replaced, but are the regulator settings adjusted back down? Best practice would have a regulator supply pressure gauge fitted and the supply pressure, minimum “no flow” and “with flow” pressure settings recorded and displayed at each regulator. This will help the operator and maintenance staff to keep the correct “with-flow” regulator setting.

 

Conclusion

This article has provided an introduction to regulators and how you can recognise the air wasted by artificial demand from their operation. Some of the causes of this have been discussed.

A following article will explain more about the different types of regulators and where and why you would use them. It will also describe what “regulator management best practice” is.

 

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

To Read Part 2 of this article visit http://www.airbestpractices.com/technology/instrumentation/managing-pressure-regulator-artificial-demand-part-2.

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