How do I select a control valve?

Select a control valve in nine stages: collect process data, calculate the required Cv or Kv, check for choked flow, flashing and cavitation, choose the valve body type, choose the flow characteristic, specify pressure class and materials, select the actuator and fail position, verify performance, then complete the specification sheet. Bore size follows from the calculated Cv, not from the pipe size.

How is Cv calculated for a control valve?

Cv is the US liquid flow coefficient: the flow in US gallons per minute that produces a 1 psi pressure drop across the valve at fully open with water at about 60 degrees Fahrenheit. For liquids, Cv = 1.96 x Ft x Q divided by N x Fl x Fp x the square root of (P1 minus P2), where Q is flow in gpm, N is the number of flow paths, P1 and P2 are upstream and downstream pressure in psia, and Ft, Fl and Fp are temperature, liquid recovery and piping geometry correction factors.

What is the difference between Cv and Kv?

Cv and Kv measure the same flow capability in different units. Cv uses US gallons per minute at a 1 psi drop, Kv uses cubic metres per hour at a 1 bar drop. Kv is approximately equal to 0.865 times Cv, and Cv is approximately equal to 1.156 times Kv.

What is the difference between linear and equal percentage flow characteristics?

A linear trim has a constant slope, so 50 percent of rated travel gives roughly 50 percent of rated flow. An equal percentage trim has a constant proportional change, so 50 percent travel gives about 50 percent of rated Cv. Use linear trim when process gain is constant, and equal percentage trim when pressure drop or system resistance varies, which is the most common case.

What does ATO and ATC mean on a fail-safe actuator?

ATO means air to open, so loss of air supply closes the valve and the valve fails closed. ATC means air to close, so loss of air supply opens the valve and the valve fails open. Pneumatic spring-return actuators are ATO or ATC as standard. Electric actuators achieve fail position through a return spring, a stored-energy mechanism, or a battery or UPS backed drive.

Should a control valve be sized for maximum flow?

Not necessarily. A valve sized only for peak flow spends most of its life near 10 percent open, where the trim sees high velocity, noise, vibration and rapid wear. Target steady operation between about 20 and 80 percent of rated travel. If maximum flow is rare, consider a split-range arrangement, two valves in parallel, or an interchangeable trim.

How do I prevent cavitation in a control valve?

Cavitation occurs when vapour bubbles form and collapse in the throat. Control it by keeping downstream pressure above the fluid vapour pressure, reducing the pressure drop across a single stage, using multi-stage or staged trim, specifying an anti-cavitation cage, selecting a larger bore and trimming the flow so velocity stays low, and avoiding high recovery factors downstream of the valve.

What is valve rangeability?

Rangeability is the ratio of maximum to minimum controllable Cv. A conventional globe valve typically offers 30 to 50 to 1, while characterized trim can reach 100 to 1 or more. Practically, most process control loops need a turndown of only about 30 to 1 between the minimum controllable and maximum rated stroke.

Support

Valve Selection Guide

Support Hits: 100

An applications guide for selecting valves

With so many types and options available, choosing the right control valve can seem daunting. Selection can be simplified by considering the process fluid, the service requirements, and how the various valves function.

The suitability of a valve for a particular application is decided by the materials used in relation to the conveyed fluid as well as its mechanical design. The table below can be used as a valve selection guide.

Conveyed Fluid	Nature of Fluid	Valve Function	Type of Trim (Disc/Ball/Plug)
Liquid	Neutral (Water, Oil, etc)	On/Off	Gate Valve
			Rotary Ball Valve
			Plug Valve
			Diaphragm Valve
			Butterfly Valve
			Plug Gate Valve
		Control valve, modulating	Globe Valve
			Butterfly Valve
			Plug Gate Valve
			Diaphragm Valve
			Needle Valve
	Corrosive (Acid, alkaline etc.)	On/Off	Gate Valve
			Plug Gate Valve
			Rotary Ball Valve
			Plug Valve
			Diaphragm Valve
			Butterfly Valve
		Control valve, modulating	Globe Valve
			Diaphragm Valve
			Butterfly Valve
			Plug gate Valve
	Hygienic (Food, beverages, drugs etc)	On/Off	Butterfly Valve
		On/Off	Diaphragm Valve
		Control valve, modulating	Butterfly Valve
			Diaphragm Valve
			Squeeze Valve
			Pinch Valve
	Slurry	On/Off	Rotary Ball Valve
			Butterfly Valve
			Diaphragm Valve
			Plug Valve
			Pinch Valve
			Squeeze Valve
		Control valve, modulating	Butterfly Valve
			Diaphragm Valve
			Squeeze Valve
			Pinch Valve
			Gate Valve
	Fibrous Suspensions	On/Off, Control valve, modulating	Gate Valve
			Diaphragm Valve
			Squeeze Valve
			Pinch Valve
Gas	Neutral (Air, Steam etc)	On/Off	Gate Valve
			Globe Valve
			Rotary ball Valve
			Plug Valve
			Diaphragm Valve
		Control valve, modulating	Globe Valve
			Needle Valve
			Butterfly Valve
			Diaphragm Valve
			Gate Valve
	Corrosive (Acid vapors, chlorine etc.)	On/Off	Butterfly Valve
			Rotary Ball Valve
			Diaphragm Valve
			Plug Valve
		Control valve, modulating	Butterfly Valve
			Globe Valve
			Needle Valve
			Diaphragm Valve
	Vacuum	On/Off	Gate Valve
			Globe Valve
			Rotary Ball Valve
			Butterfly Valve
Solids	Abrasive Powder (Silica, etc)	On/Off, Control valve, modulating	Pinch Valve
			Squeeze Valve
			Spiral Sock Valve
	Lubricating powder (graphite, talcum, etc)	On/Off, Control valve, modulating	Pinch Valve
			Gate Valve
			Spiral Sock Valve
			Squeeze Valve

Industries

Products

News & Events

Valve Experts Are Here!

Lapar S.r.l.

Address: Via Pantano 2, Milan (MI) 20122, Italy
Postcode: 20122
Phone: +39 028.2000.500
VAT N°: IT08379220968
R.E.A. MI-2021657
E-mail: sales@lapar.eu
Website: https://www.lapar.eu

Lapar Control Valve

Support

Valve Selection Guide

An applications guide for selecting valves

Related Articles

Materials of Soft-Seated Ball Valve Seats

Ball Valve Seat & Seal Material Selection Guide

Butterfly Valves

Industries

Products

News & Events

Valve Experts Are Here!

Lapar S.r.l.