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

ISO 5211 Flange - Part-turn actuator to valve attachment

Standard Hits: 14

ISO 5211 Flange - Part-turn actuator to valve attachmen

This European Standard specifies requeriments for the attachment of part-turn actuators, with or without gearboxes, to industrial valves.

This standard specifies:
• flange dimensions necessary for the attachment of part-turn actuators to industrial valves or to intermediate supports;
• driving component dimensions of part-turn actuators necessary to attach them to the driven components;
• reference values for torques to interfaces and for couplings having the dimensions specified in this standard.

Denominacion	Par max. (Nm)	d1	d2	d3	h1 max	Number bolts	d4 Metric
F03	32	46	25	36	3	4	M5
F04	63	54	30	42	3	4	M5
F05	125	65	35	50	3	4	M6
F07	250	90	55	70	3	4	M8
F10	500	125	70	102	3	4	M10
F12	1000	150	85	125	3	4	M12
F14	2000	175	100	140	4	4	M16
F16	4000	210	130	165	5	4	M20
F25	8000	300	200	254	5	8	M16
F30	16000	350	230	298	5	8	M20
F35	32000	415	260	356	5	8	M30
F40	63000	475	300	406	8	8	M36
F48	125000	560	370	483	8	12	M36
F60	250000	686	470	603	8	20	M36

Note: Bolts in tension only at a stress of 290 MPa and a coefficient of friction on 0,2 between the mounting interface

Industries

Products

News & Events

Valve Experts Are Here!

Lapar S.r.l.

Address: Galleria del Corso 1, Milano (MI), Italy
Postcode: 20122
Phone: +39 02 8200.0500
E-mail: [email protected]
Website: https://www.valveu.com

Lapar Control Valve

Support

ISO 5211 Flange - Part-turn actuator to valve attachment

Related Articles

Ball Valve Seat & Seal Material Selection Guide

Packing Material - PTFE and Graphite

ISO 5210 Flange - Multi-turn actuator to valve attachment (DIN 3338)

Pressure Rating ANSI Class vs Nominal Pressure (PN)

Industries

Products

News & Events

Valve Experts Are Here!

Lapar S.r.l.