CHARACTERISTICS OF CONTROL VALVES

Prem BabooSr. Manager (Prod)National Fertilizers Ltd. IndiaF.I.E., Institution of Engineers (India)Technical Advisor & an Expert for www.ureaknowhow.com

CHARACTERISTICS• CONTROL VALVES ARE CLASSIFIED

ACCORDING TO THEIR CHARATERISTICS ALSO

– LINEAR VALVES

– EQUAL PERCENTAGE VALVES

– QUICK OPENING VALVES

–MODIFIED CHARACTERISTICS

Control Valve Characteristics

• The relationship between flow through valve to the corresponding stem position is plotted to get valve characteristics– Inherent flow

characteristics– Installed Flow

Characteristics

–Inherent flow characteristics•It is derived from testing a valve with water as the fluid and with a constant pressure drop across the valve.

–Installed Flow Characteristics•The flow characteristic when the pressure drop across the valve varies with flow and related conditions in the system in which the valve is installed.

TYPES OF CHARACTERISTICS

• QUICK OPENING

• LINEAR

• EQUAL PERCENTAGE

• MODIFIED

QUICK OPENING• A FLOW CHARACTERISTIC that

provides maximum change in flow rate at low travels. The curve is basically linear through the first 50-60 % of travel. It then flattens out indicating little increase in flow rate as travel approaches the wide open position.

• Q = ky2

K = CONSTANT

y = Valve Opening

Q = Flow

LINEAR

• A characteristic where flow or (Cv) increases linearly with valve travel. Flow is directly proportional to valve travel.

• Q = ky

– k = constant– y = valve opening– Q = Flow

EQUAL PERCENTAGE• It is described as the type of valve

flow characteristic where for equal increments of valve plug travel the change in flow rate with respect to travel may be expressed as a constant percent of the flow rate at the time of the change.

• Q = b eay

ALTERNATELY

• Q = Q0 ef(ky)

Q = FLOW

a & b ARE CONSTANTS

y IS VALVE STROKE

MODIFIED PARABOLIC

• A FLOW CHARACTERISTIC that lies somewhere between LINEAR and EQUAL PERCENTAGE.

CONTENTS• INTRODUCTION

• CLASSIFICATION

• CONTROL VALVE PARTS

• FLOW CAPACITY & TESTING

• COMMON PROBLEMS FACED

Control valve parts

• ACTUATOR

• BODY

• ACTUATOR: A fluid -powered or electrically powered device that supplies force and motion to a valve closure member.– DIAPHRAGM:

A flexible pressure responsive element that transmits force to the diaphragm plate and actuator stem. – FORCE PLATE

The support plate which gives support to the diaphragm and exerts force uniformly

• SPRING– This is required for single acting

actuators to return to normal position when air supply is failed.

• YOKE– Yoke is the fixed connection

(mounting) between body and actuator.

• BODY– The body of the valve is the main

pressure boundary. It provides the pipe connecting ends and the fluid flow passageway. It can also support the seating surface and the valve CLOSURE MEMBER.

• BONNET– The bonnet is that portion of the valve

pressure retaining boundary which may guide the stem and contains the PACKING BOX and STEM SEAL.

• TRIM– Includes all the parts that are in flowing

contact with the process fluid except the body, BONNET, and body flanges and gaskets

• STUFFING BOX– The chamber located in the BONNET

which surrounds the stem and contains the PACKING and other stem-sealing components. 

STUFFING BOX

Cv & Kv FOR CONTROL VALVES

• flow coefficent Cv – the flow of water at 60 oF in US

gallon/minute at a pressure drop of 1 psi across the valve.

• flow factor Kv – the number of cubic meters per hour

of water at 20 oC which will flow through the valve with a pressure drop of 1 kg/cm2

• FOR LIQUIDS– Cv = Q(G/Dp)1/2

• FOR GASES– Cv = Q/1360 * {TfG / [dP*(P2)]}1/2

• FOR VAPOURS– Cv = W / 63.3 * {v / Dp} 1/2

– G = sp.gravity Tf = Flow temp. in Rank

– dP = P1-P2 v = Down stream sp.vol P1 & P2 the pressures in cf/lb

Valve Leakage Classifications

• Class I. Identical to Class II, III, and IV in construction and design intent, but no actual shop test is made.

• Class II. Intended for double-port or balanced singe-port valves with a metal piston ring seal and metal-to-metal seats. Air or water at 45 to 60 psig is the test fluid. Allowable leakage is 0.5% of the rated full open capacity.

• Class III. Intended for the same types of valves as in Class II. Allowable leakage is limited to 0.1% of rated valve capacity.

• Class IV . Intended for single-port and balanced single-port valves with extra-tight piston seals and metal-to-metal seats. Leakage rate is limited to 0.01% of rated valve capacity.

• Class V. Intended for the same types of valves as Class IV. The test fluid is water at 100 psig or operating pressure. Leakage allowed is limited to 5 X 10 ml per minute per inch of orifice diameter per psi differential

• Class VI. Intended for resilient-seating valves. The test fluid is air or nitrogen. Pressure is the lesser of 50 psig or operating pressure. The leakage limit depends on valve size and ranges from 0.15 to 6.75 ml per minute for valve sizes 1 through 8 inches.

ACTUATORS

• HOW DOES ACTUATOR WORKS AGAINST HIGH PRESSURE TO GET SHUT OFF

• TYPES OF ACTUATOR

SPRING RETURN PISTON

DOUBLE ACTING PISTON

DIAPHRAGM ACTUATORS

REVERSE ACTING DIRECT ACTING

ROTARY ACTUATOR

OPERATIONAL DIAGRAM

Multiplication of Force

F / A = P242 x .7854 = 452 sq. in.220 lb. / 452 sq. in. = .48 psi

1442 x .7854 = 16286 sq. in..48 psi x 16286 sq. in. =

7817 lb. of force

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