Pressure Measurement & Control


Pressure regulator selection criteria

November 2001 Pressure Measurement & Control

The correct sizing and selection of regulators is a complex task. A poor choice of regulator in the design phase of a plant may show an overall project cost reduction but may lead to a heavy increase in plant operational cost. Swings in regulators control pressure, due to variation in flow rates through the unit, can lead to inefficiencies within the operation of a plant and hence affect bottom line profitability. The initial price of the regulator must therefore be viewed as only a small part of the total cost of ownership.

There are two basic types of regulators on the market, direct (or self) operated and pilot-operated. Both types of regulators have their respective places within industry.

Figure 1. Typical performance curve
Figure 1. Typical performance curve

Direct-operated regulators

This is the most common unit in use within industry. The simplicity of design provides the user with lower initial equipment cost (especially on small units - typically up to 50 mm in size), ease of installation and maintenance, making it a popular choice for the user. This type of regulator can cause problems in plant performance if not sized correctly. Due to its functionality, increases in flow will cause a decay in the downstream controlled pressure. This is commonly referred to as droop or offset.

Cv values are normally stated by manufacturers, for this type of regulator, purely for the purpose of determining the flow rate through the unit, wide open, for relief valve sizing. Manufacturers published data will normally state flow rates that will be achieved for varying degrees of droop (offset). It is preferential to use this data when selecting direct operated regulators, to ascertain the amount of droop that can be expected in operation.

In an attempt to decrease the amount of droop that may be experienced in a direct operated unit, a pitot tube is often introduced into the regulator design. Due to this introduction, the regulator may actually be sensing a lower pressure downstream that actually exists and hence opens the unit wider to raise downstream pressure. This condition is often referred to as boost and can dramatically improve the droop characteristics of a regulator.

Figure 2. Performance with pitot tube
Figure 2. Performance with pitot tube

Should plant operators be experiencing unacceptable downstream pressure decay on high flow condition then various courses of action may be considered, these are:

* Adjust the spring setting on high flow to increase outlet pressure - this will cause downstream pressure to increase on low flow condition.

* Investigate the possibility of using a unit that contains a pitot tube to give the unit boost.

* Increase the size of the direct-operated regulator.

* Consider the use of a pilot-operated unit.

Figure 3. Pilot-operated regulator performance
Figure 3. Pilot-operated regulator performance

Pilot-operated units

Pilot-operated regulators have been developed to combat the obvious shortcomings of direct operated units. The pilot unit is used essentially as a pressure amplifier. This senses any change of pressure in the downstream line (P2) and supply an amplified loading pressure to the main regulator unit (PL) - this is referred to as regulator 'gain'. The use of a pilot increases the sensitivity of the unit, hence minimising the amount of droop in the system (often below 1% of controlled pressure).

The use of a pilot unit can also lead to the use of a much larger main regulator, hence giving much higher capacity availability than equivalent sized direct-operated units. Although the increased sensitivity of the unit (resulting from the high gain) can be considered an advantage, it can lead to other problems. If the system gain is too high it can lead to system instability. If this occurs then the pilot gain may have to be modified to tune the regulator to the system. Every pilot-operated regulator has a fixed and variable restriction that will allow the ratio of the two restrictions to be varied, hence changing the gain and speed of response.

Installation considerations for pilot-operated units by far exceed those for direct-operated regulators. The requirements for pilot-operated units are generally:

* An upstream tapping point and control line (or a tapping in the upstream side of the body) for pressure loading purposes.

* A downstream tapping point and control line.

* A minimum differential pressure (P1-P2) is required to drive pilot operated regulators. Should this differential not be available then an external loading source will be required.

* Due to the small orifices in the pilot, it should be ensured that the fluid used for loading purposes is adequately filtered.

Figure 4. Typical fixed and variable orifice
Figure 4. Typical fixed and variable orifice

Careful selection of gas regulators can greatly assist towards achieving optimum plant performance and hence improving bottom line profitability.

Alpret Control Specialists

(011) 249 6700

alpret.conspec@frco.com

www.alpret.co.za





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