valves used in pipelines

8/31/2015 Valves used in Pipelines

http://articles.compressionjobs.com/articles/oilfield101/4393valvespipelinesgateglobeneedleangleplugballbutterflycheck?tmpl=component&print=1&p… 1/13

Valves used in PipelinesWritten by Norrie

Sunday, 02 May 2010 18:18

INTRODUCTION

In process operations, fluids and their movement and transfer from place to place, plays a large part in the process.Fluids are contained in various types of vessels and are transported in pipelines. In order to Start or Stop fluid flow,to Control flow and to Prevent Backflow, various types of valves are installed in the piping. To prevent overpressure in piping and vessels, we use Safety Relief valves and, to isolate equipment for maintenance etc, Block valvesare installed.

Where fluids contain suspended solids scale, dirt etc…, which must be removed, Filters and Strainers of varioustypes are installed.

In steam systems, steam condensate (water), is a particular problem which can cause 'WaterHammer' in piping andequipment. (This will be explained later).

To minimize this problem, the water must be drained from the steam lines. This is sometimes accomplished byregularly 'Blowing Down' (draining the water manually), from the system. Generally, steam systems are fitted withautomatic drainage mechanisms called 'SteamTraps'.

As previously stated, valves are used for the following purposes :

1. To start and stop flow of fluids2. To regulate, throttle and control fluid flow or pressure3. To prevent backflow4. To relieve excess pressure / vacuum

TYPES OF VALVES

1. THE GATE VALVE (See Figure : 13)

This is the most common type of valve in use in industry and is used to start or stop the flow of fluids. It gives apositive shutoff when closed and is often used as a 'Block Valve' for isolating systems.

The gate valve MUST be either FULLY CLOSED or FULLY OPEN and NEVER USED TO CONTROL FLOW, asthe fluid velocity will erode the valve internals gate and body seats .. etc.

A gate valve may be of the 'RISING STEM' type as in Figure: 13 or a 'NONRISING STEM', in which the stemthreads are 'Lefthand' threads set into the gate itself as in Figure: 14.



The valve consists of a body which houses the valve mechanism the stem (or spindle), the disc(s) (or gate), and thedisc and body seats. Gate valves can be operated by a handwheel, an electric motor (MOV), or by an air powereddiaphragm, or motor, or a piston actuator.

THE RISING STEM GATE VALVE

Figure : 13

NONRISING STEM



Figure : 14

2. THE GLOBE VALVE (See Figure : 15)

The Globe valve is used where control of fluid flow or pressure is required and it can be operated in any positionbetween open and closed.

A Globe valve may be constructed with a single or double port and plug arrangement. The double port type isgenerally used in a CONTROL VALVE where accurate control of fluid is required. Due to the double valve plugarrangement, the internal pressure acts on each plug in opposition to each other, giving an internal pressure balanceacross the plugs. (See Fig: 20 on page 42 – Control Valve)

This gives a much smoother operation of the valve and better control of the process. Some control valves are 'ReverseActing'. Where a valve normally opens when the plug rises, in the reverse acting valve, the valve closes on rising. Theoperation of the valve depends on process requirements. Also depending on requirements, a control valve may be setto open or close, on air failure to the diaphragm.



Figure : 15

3. THE NEEDLE VALVE (See Figure : 16)

This is simply a variation of the Globe valve and, as its name implies consists of a narrow, tapered plug and portarrangement. The needle valve is, in itself, small in size and is used for very fine and normally manual, control offluid flow.

4. THE ANGLE VALVE (See Figure : 17)

Another variation of the Globe valve which is often used for compressor recycle systems. It is also often used fordirty fluids. In this valve, the fluid flow enters from below the plug and passes through an angle of 90° to the outlet.An angle valve may be manually or automatically controlled.

5. THE PLUG or BALL VALVE (See Figure : 18)

These valves give a more streamline flow in their operation. The valve port is about the same size as the internal pipediameter. They are operated simply by turning a handle through 90°. They can give a fairly tight shutoff and somedegree of control.

6. THE BUTTERFLY VALVE (See Figure : 19)



The Butterfly valve is designed to give full flow with some control capability. Normally, this type of valve does notgive positive shutoff but, presentday technology, using new Polymers and higher temperature ratings, has enabledthe construction of butterfly valves having a 'Bubbletight' shutoff.

7. CHECK or NONRETURN VALVES (NRV's) (See Figure : 20)

These valves will allow fluid flow in ONE DIRECTION ONLY. Their purpose is to prevent backflow for examplein pump discharge lines and where backflow is undesirable. When flow stops, the weight of the piston, ball orflapper closes the valve. If the pressure downstream of the valve, is higher than the upstream side, the valve will closemore firmly and backflow cannot occur.

There are two main types of check valve :

1. The 'LIFT' type. (Spring loaded 'BALL' & 'PISTON' Types).2. The 'SWING' (or Flapper Type).

THE NEEDLE VALVE

Figure : 16

THE ANGLE VALVE



Figure : 17

THE PLUG VALVE & BALL VALVE



Figure : 18

THE BUTTERFLY VALVE (SIMPLIFIED)



Figure : 19

CHECK or NONRETURN VALVES (NRV's)



Figure : 20

8. SAFETY RELIEF VALVES (See Figure : 21)

Safety valves are installed on equipment to prevent overpressure (or excess vacuum), which could cause damage,rupture or collapse of the equipment.


http://articles.compressionjobs.com/articles/oilfield101/4393valvespipelinesgateglobeneedleangleplugballbutterflycheck?tmpl=component&print=1&… 10/13

Where the type installed is to prevent excess vacuum, they are normally referred to as 'Vacuum Breakers'.

A safety valve will have a lifting set pressure higher (about 1.5 times) than the operating pressure of the system butwell below the design maximum pressure allowed.

In the conventional type, the pressure of the valve discharge line will affect the set pressure of the valve by giving abackpressure on the valve plug.

In the bellows type, the bellows isolates the plug from the backpressure and therefore there is no effect on the valvesetting.

The following diagram is of a conventional safety relief valve. All safety valves have the same purpose of protectingequipment from overpressure. Different makers have their own designs.

SAFETY RELIEF VALVE

Figure : 21

VALVE ACCESSORIES



1. STUFFING BOX and PACKING (See Figure : 22)

Packing is a means of preventing leakage around the valve stem to atmosphere. The packing is placed in the 'StuffingBox' around the stem. It is then held in place and compressed by the 'Gland' and 'Gland Follower' to give a leaktightseal.

However, having the packing too tight can cause the operation of the valve to be stiff. When tightening down on thepacking does not stop leakage or causes valve operation to be stiff, the packing should be replaced.

In some larger valves, a 'Lantern Ring' is inserted between the turns of packing and is placed opposite to a lubricationpoint where grease can be injected in order to ease the valve operation.

Figure : 22

2. VALVE OPERATORS (See Figure: 23)

The most common valve operator is the 'Handwheel'. Other methods of valve operation can be as follows :

A. A DiaphragmHere, the valve is operated by an air powered diaphragm. These are called 'Pneumatic Control Valves'. The airpowered actuator can also take the form of a 'Piston' rather than a diaphragm. The operation of the valve, dependingon requirements, may be 'Air to Open' (AO) or 'Air to Close' (AC).



B. An Electric MotorThis is called a 'Motor Operated Valve' (MOV) and may be installed to allow valve operation from a remote locationand may then be referred to as a 'Remote Operated Valve' (ROV).

C. A SolenoidThis method of valve operation is used for a small valve placed in an air supply line to a shutdown device. Thesolenoid will be activated on an electrical signal from an automatic trip system and will open or close depending uponits function.

Figure: 23 Diaphragm Operated Valve

valves used in pipelines

Documents