Valves: How do they Work and Why are they Important?

At their most basic, valves are devices that control, direct or block away flow within a system or process. They often feature a range of characteristics that provide options for most process applications. From the perspective of a Chemical Process Engineer, the following work process can be used to determine what type of valve is needed for an application. Typically, Chemical Process Engineers design ‘control’ valves, but this process works for block or ‘on/off’ valves as well.

By Tim Goedeker – Tricord Consulting

Criteria for Valve Selection

The selection of valve types requires an understanding of valve function and service characteristics (process design conditions).

Valve Function

Valves perform many functions and are used in many applications in process industries, including:

• Isolation – Starting/stopping (isolat­ing or ‘on/off’) The flow of a fluid (hy­drocarbons, oil & gas, steam, water, acids) through a pipe (example: gate valve, ball valve, butterfly valve, or plug valve).
• Control – Regulating or controlling the flow of a fluid through a pipe (exam­ple: globe valve, butterfly valve, ball valve, or plug valve).
• Non-Return – Protecting a piping sys­tem or a device (pump, motor, tank) from back-pressures/back-flow of fluid (swing-check valve, lift-check valve).
• Safety Relief – Protecting a piping sys­tem or a device (pump, motor, tank) from overpressures or vacuums (safe­ty or pressure relief) or regulating the pressure of a process (pressure reduc­ing valve).

Figure 1 highlights some common valves in industry. Globe and ball valves can be used for isolation as well as control of the fluid. While many valves accomplish similar action, the way they do it mechan­ically can vary. How a valve opens and closes will not only impact the system hydraulic performance but will also de­termine how much control the operator has over the flow and how quickly the valve changes the flow.

Valves have a range of characteristics and comply with a variety of standards, see Figure 2 & 3. Common types are:

• Globe – Globe valves are typically ap­plied in varying ‘control’ of flow op­erations.
• Gate – One of the most common valve types. These are used most often in fully open or closed positions and typi­cally not used for flow control.
• Ball – Typically includes quick-acting 90-degree turn handles. These valves use a ball to control flow which provides easy ‘on/off’ control. Generally accepted as easier and faster to operate than gate valves.
• Butterfly – A valve with a compact wa­fer-type design, the butterfly is also a quick-acting rotary motion valve ideal for tight spaces.
• Check – Used to prevent fluid backflow, these valves are self-activated which al­lows the valve to open automatically when flow passes through the valve in the intended direction or close if the flow reverses.
• Pressure Relief – Used to protect equipment from overpressure and im­prove process safety. These valves are spring-automated and when engaged (opened) release fluid to help return a system to the desired pressure during an over-pressure situation.

Service Characteristics – Valve Design Criteria

Valve design conditions are the primary way to find valves which fit a project application or an existing process op­eration. The first thing to consider is the valve size based on the design flow rates. Choosing a valve that does not provide the design flow rate can lead to flow control and system operability problems. If the valve is too small, it could cause reduced flow downstream while creating back-pressure upstream. If the valve is too large, the flow con­trol could be significantly reduced with­out adequate pressure drop created by the valve.

Depending on the design conditions, the materials of construction may be a very important aspect in ensuring safe opera­tion, and valve maintainability.

Valves with ‘on/off’ functionality seem simple, but that application must be considered as well. For example, the valve may need to open and close very quickly, or it may be critical that the valve open and close slowly to avoid shocking the piping system or to pre­cisely control flow.

The key design parameters for selecting a valve that must be addressed are:

• Is the valve used for flow ‘control’ (con­trol valve) or to be an ‘on/off’ valve (block valve)?
  • Are there any environmental valve ‘leak’ standards for this valve?
    • Do LDAR (Leak Detection and Repair) standards apply to the process op­erations for this valve?
• What are the min/max flows for the valve?
• What is the allowable pressure drop across the valve?
• What are the min/max operating tem­perature & pressure for the fluid pass­ing through the valve? Include non-routine operations; nitrogen purge, steam out, etc.
• What is the process fluid?
  • The fluid typically is one of the fol­lowing: liquid (including two-phase); gas/vapor; steam; slurry, solids, which can be further subdivided by characteristics of the ‘service’ for the fluid:
    • Clean – Fluids with little to no sol­ids or contamination
    • Dirty – Fluids with suspended solids that may impair the per­formance of the valve
    • Slurry – Fluids with substantial suspended solids
    • Toxic – Fluids such as chlorine, hydrofluoric acid, hydrogen sul­fide, CO, phenol, etc.
    • Corrosive – Fluids such as sulfu­ric acid, acetic acid, hydrofluoric acid (HFA), wet acid gas (wet CO2), sour gas (wet H2S), and chlorides. The corrosive nature of the fluid will greatly influence the materials of construction for the valve internals
    • Viscous – Fluids such as high-vis­cosity oils (lube and heavy fuel oil) and wax crude, gels, and pastes.
• Special attention should be given to the check valves, a sluggish response may cause operating difficulties and even hazardous conditions.

Valve Design Data ‘Data Quality’ is Important

Gathering accurate design data is a chal­lenging activity typically requiring field work, records review (if they exist) and process simulation work. However, it is one of the most important. Poor quality design data will lead to poorly specified valves and potentially poor operations af­ter installation of the valve(s). A sketch of the process piping, an understanding of the process hydraulics, including pump curves, are helpful tools when reviewing the design data and choosing the proper valve.

Final Thoughts

I cannot overstate this. Just like cutting a piece of lumber, “measure twice, cut on­cecut once, and verify!” After all design conditions have been determined, and the valve has been chosen, step back and “re-verify” that the valve(s) chosen are correct for the conditions.

Choosing the right valve might seem complex. However, by starting with gen­eral characteristics — such as valve de­sign, and process design conditions – an individual can quickly limit the options to determine the best valve(s) for the proj­ect or process needs.