Valves

D:\SO\wr\IHRDC-Modules\Module-03-Valves.doc

16 May 2000

ADMA-OPCO

On-site Training Course

Production / Process

Module 3

VALVES

Gap Elimination Program

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Production / Process

Module 3

VALVES

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TABLE OF CONTENTS

1. Introduction______________________________________________________ 5 2. Common Valve Types ______________________________________________ 5 3. Valve Selection___________________________________________________ 12 4. Control Valves ___________________________________________________ 14

4.1 Control Valve Selection_________________________________________________ 14 4.2 Control Valve Sizing ___________________________________________________ 21 4.3 Valve Characteristics __________________________________________________ 22

5. Valve Actuators __________________________________________________ 25

5.1 Actuator Types________________________________________________________ 25 5.2 Methods of Actuation __________________________________________________ 25

6. Pressure Relief Valves_____________________________________________ 34

6.1 Safety Valves _________________________________________________________ 34 6.2 Relief Valves__________________________________________________________ 35 6.3 Safety-Relief Valves____________________________________________________ 36 6.4 Vacuum Breaker Valves ________________________________________________ 36

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OBJECTIVE Upon completion of this Module, the developee will be able to correctly identify the following: Common valve types Classification of valves by function Control valve types and principles of selection Relief valves, safety valves and safety relief valves Types of valve actuators and principles of operation.

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VALVES 1. Introduction

Valves are extensively used in oil field and plant operations. Because of this extensive use, their selection and maintenance has become extremely important for production and process personnel. The aim of this section is to help engineers and other production personnel to understand the basic design of the various types of valves used in oil field operations.

2. Common Valve Types The common valve types are tagged with names that are derived from either the design of the fluid control element, the primary function of the valve, or the construction of the body. The proper selection of valves depends to a large extent on understanding the distinctive features of the basic types. Valves are used to control flow. A valve may regulate (throttle), stop or start the flow. There are many different types and sizes of valves. All valves have common characteristics

such as inlet and outlet ports and some way of closing. Valves are made of different materials, such as cast iron, stainless steel, bronze and plastic. The type of valve used in a system depends upon its application and the fluid to be

transferred. Valves are made in the same sizes as pipes so hat hey will match. The regulating valve opens or closes as necessary to keep pressure constant. The throttle

valve is used to control the flow velocity. A valve used to start or stop fluid may perform other functions such as; regulating or throttling.

The ball, globe, gate, butterfly, plug, needle, check and quick opening or fast acting valves

are common valves. Other types of specialised valves like relief and safety valves are also used. Valves may be operated manually, mechanically, electrically, hydraulically or pneumatically.

Gate Valve It is used to stop the flow of a fluid by a gate made of metal. The fluid flow is straight through the body of the valve. The pressure loss through this valve is small.

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Globe Valve The fluid flowing through the globe valve changes direction. The flow is controlled by a disc moving up or down. Globe valves can be adjusted with few turns and they are preferred for frequent opening and closing applications. They provide an excellent means of throttling flow.

Angle Valve

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It has same basic design as the globe valve. An angle valve reduces the number of joints required if a valve is needed near a right-angle change in he direction of a pipeline.

Needle Valve It is similar to the globe valve except that it is very accurate at metering the amount of fluid passing through and is used as a control valve for most instruments.

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Ball Valve It has a cylindrical hole through it. When the hole is in line with the pipe, flow is permitted. When the ball is turned 90 degrees the solid surface of the ball blocks the flow. The pressure in the line helps to keep the ball valve closed. Fluid can flow through it in either direction.

Plug Valves The blocking element of the valve has an open rectangular or round channel through it. The plug fits into the body of the valve and offers either its open part or its solid side to the fluid, a quarter turn is required to change it from one position to the other.

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Butterfly Valve It has a straight-through flow pattern. A plate called the Butterfly controls the flow passing through the butterfly valve. This plate can be rotated so it blocks the flow through the valve or it can be opened to allow the flow to pass. Butterfly valves are most commonly used as throttle valves.

Check Valves The purpose of the check valve is to permit fluid to flow through a line in one direction only. When the fluid is moving in the proper direction, the valve remains open. If the fluid reverses, then the valve is closed automatically by the pressure of the fluid acting against it. Check valve types are swing check, lift check and ball check.

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Swing Check Swing check valve closing element swings open and shut like a trap door. This valve offers little resistance to flow when it is in the open position. Swing check valves are generally used in piping applications where very little pressure drop is an important consideration. Lift Check These valves are either "Horizontal lift" or "Vertical lift" check. Ball Check These valves are designed to handle thick fluids, and for service where scale and other particles are present. They are made in vertical, horizontal, and angular designs. Ball check valves are recommended for applications of rapidly changing fluid flow because of their quiet operation. The ball moves rapidly up and down as it opens and close. The ball rotates to equalise wear on both the ball and the valve seat. Check valves may be made of brass, cast iron, or steel.

Diaphragm Valves The working parts of the diaphragm valves are always separated from the fluid flow. A flexible diaphragm separates the working parts from the seat. The valve is used where there are problems with corrosion, contamination, abrasion, clogging, and leakage.

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Quick-Opening Valves They are modified to provide a quick on/off action. In place of the usual threaded stem and control wheel, lever is used to operate the valve.

Safety Valves They provide a rapid means of getting rid of excess pressure before serious accidents occur Safety valves are used with gases and steam. These valves function by popping wide open at a preset pressure. Safety valves remain open until pressure has dropped slightly lower than the normal line pressure and the valve snaps shut instantly.

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Relief Valves They are used with liquids and start to open at a preset pressure, as he line pressure increases. The valve opens further until it has reached its maximum travel. As the pressure drops the valve starts to close, at below its preset pressure limit it is completely closed.

3. Valve Selection The first step in selecting a valve is to consider its function. Valve function, more than any other factor, narrows down the choice of the valve. A classification of valves by function is given below. On/Off Service - Gate Valves - Plug Valves - Ball Valves Throttling Service - Globe Valves - Angle and Y Valves - Needle Valves - Butterfly Valves - Diaphragm Valves

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Pressure Control - Safety Valves - Relief Valves - Safety Relief Valves - Back Pressure Regulating Valves - Pressure-Reducing Valves Other Controls - Solenoid Valves - Temperature-Regulating Valves - Flow-Regulating Valves Prevention of Back Flow - Check Valves

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4. Control Valves In process systems, the final control element is normally a pneumatically actuated control valve, which is used to regulate the flow of a fluid. It provides the necessary power to translate the controller's output to the process. Pneumatics are used because of the original popularity of pneumatic control systems and the comparatively low operating pressures used.

Of the basic components of a control loop the control valve is subject to the harshest conditions. A control valve is also the most expensive item and the most prone to incorrect selection.

4.1 Control Valve Selection

The choice of control valve will depend upon the application. i.e. flow control, ESD etc. The main factors to take into account are the valves' ability to regulate flow, the pressure loss/recovery when fully open and the shut-off leakage. For instance a globe valve gives good flow regulation, has poor pressure recovery at high flow rates and does not give tight shut-off, whereas a ball valve has poor flow regulation characteristics, low pressure loss at high flow rates and has the advantage of tight shut-off. Space is another factor that can come into the equation.

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The main selection factors of control valve type can be summarised as follows: The process requirements: The inlet pressure, the differential pressure across the valve, the maximum and minimum flows and the degree of shutoff required.

(a) Basic elements (b) Operation

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Actuator

Body And Plug

Motive Power

Process fluid

Motive Power

Moves actuator

Moves plug

Mainpulates

fluid stream


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4.2 Control Valve Sizing

Normally a valve is designed to handle its maximum flow when it is at 75% open. Making the valve too big or too small would be detrimental to the operation of the valve and the loop. Valves should not operate below the 1 0% open position or above the 90% open position.

Suitable flow characteristics to match the process Fail safe mode Proper choice of valve body type and accessories. Correct installation

Valve shutoff There are six classes of valve leakage

Class 1 no test Class 11 0.5% of rated valve capacity Class Ill 0.1% of rated valve capacity Class IV 0.0 1 % of rated valve capacity Class V 0.005% mL/min of water per inch of port diameter per psi differential Class VI bubble tight If tight shutoff is required, it is good practice to provide a tight shutoff isolation valve in series with the throttling valve. The soft seat in a throttling valve will need to be frequently replaced if it used to carry out tight shutoff.

Valve Trim

Valves control the rate of flow by introducing a pressure drop across the valve trim. The

valve trim consists of the plug and seat (these are usually sold as matched sets, which have been ground to a precise fit in the fully closed position).

All valves have a throttling action that causes a reduction in pressure. If the pressure increases again too rapidly air bubbles, entrained in the fluid implode, causing rapid erosion of the valve plug and seat surfaces. This process is known as cavitation.

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4.3 Valve Characteristics

The flow characteristics of a valve defines the flow behaviour of the valve as it is operated through its rated stroke. It is usually shown as a plot of valve opening or lift (% of maximum) against flow (% of maximum). The characteristics quoted by the manufacturers are theoretical or inherent flow characteristics obtained for a constant pressure drop across the valve. The actual or installed characteristics are different from the inherent characteristics, since they incorporate the effects of line losses acting in series with the pressure drop across the valve.

The three main types of characteristics are:

Quick-opening Linear Equal-percentage

4.3.1 Quick-Opening

This is the characteristic of the level seated disc or plain flat disc type of valve plug. With this type of valve, near maximum flow rate occurs at relatively small stem travel, approximately 35%, after this value there is little change in flow rate.

4.3.2 Linear

The linear characteristic shows that the relationship between valve opening and flow at constant pressure drop is a straight line, i.e. equal increments of lift should cause equal increments of flow (30% flow at 30% open, 70% flow at 70% open).

4.3.3 Equal Percentage

As the name implies, an equal percentage valve is one in which equal increments of lift cause equal percentage increments in flow. In practical terms this means that the valve will give accurate throttling control over its lower range and rapidly increasing capacity as it approaches the wide open limit. This implies that the sensitivity of the valve increases with valve opening. These valves are used when major changes in pressure occur across the valve and where there is limited data regarding flow conditions in the system.

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4.3.4 Use of Valve Characteristics.


Normally the choice of valve characteristic required by the loop is established by carrying out a dynamic analysis of the control loop but there are rules of thumb that can be applied to general situations.

Linear trims are used in situations, such as level control, where the pressure

drop across the valve is constant. Equal percentage trims are best used in flow situations where the pressure

drop across the valve will vary as the flow goes from its minimum value to its maximum value. This is especially true on pumped systems.

Quick opening valves are useful in by-pass or re-cycle lines where a basic on-

off control of flow is required.

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5. Valve Actuators 5.1 Actuator Types

The actuator provides the power to vary the orifice area of the valve in response to a signal received. An actuator is classified according to its input signal and its output to the power element (valve etc.). It maybe mechanical, pneumatic, electrohydraulic, or electro-pneumatic. It can receive its operating air signal from a pneumatic controller either directly or through an auxiliary device such as a valve positioner. The input signal usually varies between 0.2 and 1 Bar, acting on the effective area of the actuator. This results in a force which is used to position the control valve. The diaphragm type of pneumatic actuator consists of a flexible diaphragm in a pressure tight housing attached to the valve stem. The diaphragm acts as a pressure seal between the upper and lower chambers. The conventional type diaphragm consists of a flat piece of flexible material of circular shape and is pre-moulded to prevent crimping or stretching. The upper and lower cases of the diaphragm housing are usually held together by bolting or clamping. The diaphragm actuator is available with either a straight stem or a lever for connection to the final control element. These are referred to as thrust and lever type respectively. The thrust type is used where the actuator stem can be directly coupled to the valve stem such as with globe or gate valves. The lever type is used with final control elements such as butterfly valves.

5.2 Methods of Actuation By use of the correct failure mode of the actuator, the valve can be made to FAIL TO OPEN or FAIL TO CLOSE depending on the fail-safe requirements of the process. The main actuator types available are: 5.2.1 Spring Type Diaphragm Actuator

In this type of operator, a spring opposes the force of the air pressure acting over the effective area of the diaphragm. It provides a given position of the valve stem for a specific input signal pressure received from the controller. This design is inherently fail safe, reliable and has few moving parts. It has however limited power, limited seat shutoff capabilities and is slow to operate.

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5.2.2 Piston Type Actuator

The air piston provides high torque or force and has a fast stroking speed. It provides a high power to weight ratio, has few moving parts and an excellent dynamic response. It can handle high differential pressures and provides high shutoff capability.

5.2.3 Valve Positioners

In theory, a control valve should respond quickly to small output changes from the controller. However, if the controller output pressure is very small, the actuator may not be able to develop sufficient force to position the valve correctly and/or fast enough for good control, This failure could be caused by stiction between the stem and the valve packing, unbalance of the valve plug due to the hydrostatic forces of the process fluid or hysteresis within the valve itself.

This creates two main problems:

It takes a greater force to initially move the valve in any direction, so causing a dead

spot. Once the valve is moving, the initial force applied will cause the valve to accelerate

and this in turn could cause overshoot and instability in the process

Therefore instead of sending the signal from the controller directly to the valve diaphragm, the signal is passed to a slave controller, with its own air supply, known as a positioner.

A positioner is fitted to a valve in such a way that it can monitor the valve position and adjust its output signal until the valve is at the position required i.e. the inclusion of a positioner provides closed loop control of the valve position.

Positioners can provide the following benefits

accurate positioning of the valve stem ability to change the valve characteristics split the operating range of two or more valves increase the speed of response reverse the action of a valve

5.2.4 Solenoid Actuators

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These are only used on small control systems where on-off control is required. Mostly they are found in the form of three way valves on the signal lines from the controller to the valve for ESD use. On removal of the power the valve will disconnect the controller from the valve and vent the air in the valve to atmosphere.


Note It is necessary to take the flow characteristics into account when making a choice of valve. Choosing the wrong valve can result in a process that is difficult to control over a range of conditions.

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Valve Failure Mode with Different Valve/Actuator Setups

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6. PRESSURE RELIEF VALVES

Relief valves operate when the pressure in a system or a vessel rises above or drops below the safe limit. Three types of pressure-relief valves are used: safety valves

relief valves

safety-relief valves All three types of PZVs perform the same basic function. They relieve excess pressure in a system. Most PZVs relieve pressure to the blowdown system. Some relieve pressure to the atmosphere. This occurs when PZVs are at the top of Very tall columns, or to discharge air or steam. In the plant, pumps, boilers, heat exchangers and vessels operate under pressure. These can operate safely a little above normal operating pressures. However, there is a limit to how much pressure a piece of equipment can withstand. Relief valves can safely vent excess pressure. 6.1 Safety Valves High-pressure steam or gas systems use safety valves. A safety valve is a pressure

relieving device. Vapors can be compressed, so safety valves must open quickly when the set pressure is exceeded. A safety valve makes a sharp popping sound when it opens.

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6.1.1 Major Parts of a Safety Valve Figure 17 shows the major parts of a typical safety valve. A safety valve operates

automatically by static gas or vapor pressure upstream of the valve. Each valve has a hand lever so you can also operate it manually.

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6.1.2 How Safety Valves Function

Gas pressure upstream of the valve presses on the disc. The disc connects to a spring actuator. At the set pressure, the spring starts to compress and the valve pops open.

6.2 Relief Valves

A relief valve is another kind of automatic pressure relieving device. Relief valves relieve liquid pressures. Liquids cannot be compressed. So releasing a small amount of liquid brings the pressure down to safe levels. A typical relief valve relieves only 10% of its opening pressure.

The figure shows a relief valve. The main parts are similar to a safety valve. Relief valves do not pop like safety valves. They open gradually as the upstream pressure increases.

Relief Valve


6.3 Safety-Relief Valves You will see safety-relief valves on all process equipment. These valves can operate

either as safety valves on gas or vapor systems, or as relief valves on liquid systems. The main parts are similar to a safety valve or a relief valve. Safety-relief valves can

operate as either type. 6.4 Vacuum Breaker Valves Some tanks in the tank farm have vacuum breaker valves. These let air into the tank to

prevent it from pulling a vacuum. If a partial vacuum develops inside the tank, it may rupture. Vacuum breaker valves open at a very low positive pressure. This takes air into the tank. Inside and outside pressures remain the same.

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1. Introduction 2. Common Valve Types 3. Valve Selection 4. Control Valves 4.1 Control Valve Selection 4.2 Control Valve Sizing 4.3 Valve Characteristics 5. Valve Actuators 5.1 Actuator Types 5.2 Methods of Actuation

6. PRESSURE RELIEF VALVES 6.1 Safety Valves 6.3 Safety-Relief Valves 6.4 Vacuum Breaker Valves

Valves

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