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

[email protected]

www.alpret.co.za





Share this article:
Share via emailShare via LinkedInPrint this page

Further reading:

VEGA handles the pressures of water treatment systems
VEGA Controls SA Pressure Measurement & Control
A water treatment system for a major metropolitan area in the Midwestern United States demands careful monitoring and management of processes across its sprawling network. Choosing VEGA for its process automation needs meant more than just obtaining precise and reliable pressure sensors.

Read more...
Monitoring the health of systems
SA Gauge Pressure Measurement & Control
Pressure and temperature gauges are vital instruments in various industries. However, they are susceptible to failures that can compromise their accuracy and reliability. When these gauges fail, the consequences can be severe.

Read more...
Enhancing beer brewing precision
VEGA Controls SA Pressure Measurement & Control
Brewing beer is a complex process that requires utmost precision in every step. To ensure precision, it is crucial to measure and control the pressure and point levels in the wort kettle and hops dissolver. Using VEGA instrumentation can offer numerous benefits to breweries.

Read more...
Greener mining through water management strategies
VEGA Controls SA Pressure Measurement & Control
Water scarcity is a pressing concern in South Africa, leading to the establishment of stricter regulations on water discharge and management. To promote greener mining operations that minimise environmental impact and optimise resource utilisation, responsible dewatering practices are essential.

Read more...
Pressure sensors for hydrogen applications
Instrotech Pressure Measurement & Control
Keller manufactures isolated piezoresistive pressure sensors. The great advantage of the piezoresistive measuring principle is its high stability.

Read more...
A new approach to milk processing
VEGA Controls SA Pressure Measurement & Control
Ensuring the quality and safety of milk throughout the production process is of the utmost importance in the dairy industry. Process instrumentation plays a vital role in this stage by providing accurate measurements of level, pressure and point level to guarantee optimal storage conditions.

Read more...
OEM high-pressure transducer with maximum long-term stability
Instrotech Pressure Measurement & Control
Instrotech is offering KELLER 10LHP, the flagship OEM pressure transducer in the KELLER product portfolio, which exemplifies the highest standards.

Read more...
Sensor technology for cartonboard machine for paper and packaging
VEGA Controls SA Pressure Measurement & Control
Paper and packaging specialist, LEIPA undertook a comprehensive modernisation project, combining three stock preparation lines into one, and making investments in new plant components and extensive automation technology. Because of the good experiences the company had already had with VEGA sensors, those responsible for planning and maintenance decided in favour of the VEGABAR 82 pressure transmitter as the standard instrument for level and pressure measurement in the new stock preparation system.

Read more...
Pressure measurement without risk of contamination
WIKA Instruments Pressure Measurement & Control
Injectables are among the most sensitive products in the pharmaceutical industry. Sterility is essential during production. For this reason, a manufacturer of biopharmaceuticals decided to implement the pressure monitoring of its processes with hygienic instruments from WIKA.

Read more...
Pressure monitoring for oil and water tempering machines
Instrotech Pressure Measurement & Control
Temperature plays a significant role in many manufacturing processes. In such cases, a tempering machine is used to maintain the manufacturing process at the desired temperature. Keller supplied PR-21Y piezoresistive pressure transmitters to a client specialising in developing and producing oil and water tempering machines.

Read more...