Chemical plant

Understanding control valve basics

When selecting a two-way flow control valve, which factors come into play? Quick, easy and simple-to-source might be tempting. However, considering these factors will increase performance, reliability and product longevity of your control valves.
 
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Article by Albert H. Gunnarson
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First of all, the medium. This can determine materials of construction for erosive- or corrosiveness. The fluid temperature can also influence material selection and valve assembly construction choices.

Furthermore, the inlet pressures and presumed pressure drops for each flowing condition should be considered. This can and should determine the control valve type, see the various types below.

Apart from the control valve type, inlet pipe size (diameter) and required flow rates should also be taken into consideration. This can determine a relative valve range size that one should be focused on along with rangeability requirements for turn-down (maximum flow vs minimum controllable flow).

These are all important considerations before focusing on control valve selection. Others no doubt will include successes and failures previously used, the space / location of the valve to be fitted, along with actuation and accessory requirements. To get started, once you have tabulated the answers to all your questions you can begin to review the characteristics and best uses for all control valve types.

Control valve trim styles

For the rotary 1 ⁄4-turn control valve types, there are not many trim style choices of note that influence initial valve selection. However, for the globe style control valve, there are a few different trim styles to discuss for proper application selection:

Single seat unbalanced

Lowest cost choice for small to medium flowing differential pressures. May become impractical for larger sized globe valves for actuation cost.

Single seat, cylinder-balanced

Lowest cost solution for medium flowing differential pressures up to about 450°F, with clean fluids, for the elastomeric cylinder seal.

Double seat, balanced

Balanced valves offer higher close-off pressures with smaller actuators which can save money, but at best the double-seat design is only rated for ANSI Class III leakage, and that is only at room temperature.

Due to the valve stem typically being made of a different material than the valve body, the inherent growth rate in temperature between these two metals will induce greater leakage as fluid temperature increases from ambient (as with steam). As such, a double seat valve is never chosen for steam, but can excel for dirty or turbid water compared to other balanced designs.

Single seat, cage-retained-seat (caged), unbalanced

Medium cost choice for small to medium flowing differential pressures. May become impractical for larger sized globe valves for actuation cost. Better reliability and serviceability of the trim.

Single seat, cage-retained-seat (caged), balanced

Medium cost solution for medium flowing differential pressures up to about 450°F, with clean fluids, for the elastomeric cylinder seal. Better reliability and serviceability of the trim. Some MFG’s will offer higher temp seals but those are usually at ANSI Class II leakage.

Single seat, cage-balanced, multi-stage trim

Highest cost for severe-service, high-pressure drops. Comes in two major categories: (1) for liquids, referred to as anti-cavitation trim in 1, 2 or 3 – stage reductions or (2) for gases and steam, referred to as aerodynamic trim which ‘tunes’ the frequency of the noise to the human inaudible spectrum. Both choices are expensive and, for the inherent control valve reductions, generally require larger valves. There are other design solutions to consider as alternates, like multiple control valves sequenced to keep pressure drops above the vapor pressure, and silencers and noise absorbing insulation blankets to address aerodynamic noise.

To conclude, when it comes to control valve selection, the type of control valve you choose is vitally important to overall valve and system performance, including reliability and longevity.

Whatever your choices are, it is always recommended to have your full process conditions verified against your control valve choice by a computerized valve sizing and selection software with documented reports for historical verification.
 

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