Albert H. Gunnarson the process of selecting the right steam control mechanism, as not properly selecting the correct mechanism can cause significant damage and harm to personnel and equipment
If you suddenly have the need for a two-way, flow control valve – from a valve-selection standpoint, what kind of questions do you ask yourself?
Many perhaps don’t ask any questions and may make rash decisions for what was quick, easy and simple to source. The fact is, if performance, reliability and product longevity are desired outcomes, then asking and answering a series of questions can be relevant and important.
What is the fluid? This can determine materials of construction for erosive or corrosiveness.
What is the temperature of the fluid? This can also influence material selection and valve assembly construction choices.
What are the inlet pressures and presumed pressure drops for each flowing condition? This can and should determine control valve type.
What is the inlet pipe size (diameter) and required flow rates? 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 types
- Modulating, Sliding Stem, Globe Style Control Valve. Best all-around choice for all non-viscous fluids applied in a wide range of flowing differential pressure drops. All sizes – larger valves can be expensive.
- Eccentric Plug, Rotary Style Control Valve. Best all-around choice ¼-turn choice for all fluid types and slurries applied in a wide range of flowing differential pressure drops. Economical compared to globe valves in larger sizes but will have higher aerodynamic noise than globe valves.
- ¼-Turn, V-Ported Concentric, Segmented Ball valve. Good, low-cost choice that is best reserved for low flowing differential pressures. Claimed high turn-down is often not usable in real applications. Will have inherently low valve authority.
- ¼-Turn, V-Ported Concentric, Full Ball valve. A smaller version of the segmented-ball, typically in sizes up to 2-in, with the same characteristics as its cousin.
- ¼-Turn, Concentric, Full-Port, Ball valve. This is not a good choice as a control valve, though some may try to modulate with it. Only good for low flowing differential pressures, best when used in continuously modulating applications between 25% and 75% of stroke. Will have inherently low valve authority. Very low cost – typically up to 2-in.
- ¼-Turn, Butterfly valve. This is not a good choice as a control valve, though some may try to modulate with it. Only good for low flowing differential pressures, best when used in continuously modulating applications between 25% and 75% of stroke. Will have inherently low valve authority. Very low cost – typically 2.5-in and larger.
Control valve trim styles
For the Rotary / ¼-Turn control valve types, there are not many trim style choices of note to discuss 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 do 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 Cv 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.
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 computerised valve sizing and selection software with documented reports for historical verification.
Albert H. Gunnarson is the Vice President of Sales, Marketing & Product Development at Warren Controls.