Relief Valves

Valves [Lesson 6]

5. Relief Valves
Purpose: Relief valves are used to protect systems from over-pressure or to control processes by allowing flow to commence when a certain pressure has been reached.
Operation: Refer to Figure

A spring holds the valve disc in place against the seat. The valve, therefore, will not open until the force exerted on the valve disc by the fluid pressure exceeds the force exerted by the spring. When this occurs, flow can take place through the outlet port until the fluid pressure is reduced to below the valve operatingpressure. The spring force will then release the valve.

Relief valves operate automatically and are usually pre-set to a specified relief setting by the manufacturers or adjusted when in use, if required. Re-calibration is then required

Butterfly Valves

Valves [Lesson 4]


4. Butterfly Valves

Purpose: Butterfly valves are used for controlling flow and can act as a shutuoff ~va1ve, if the sealing arrangement is designed accordinglyThe disc or wafer rotates about a vertical axis and can be turned through 90º.

The disc seals against the opening to cut off flow and can be positioned at a point between fully closed and fully open as required.

Some butterfly valves have a direct acting lever, others are operated through gearboxes when finer control is required. In most cases, a clockwise movement will close the valve

Globe Valves

Valves [Lesson 3]

2. Globe Valves
Purpose Globe valves are used to control flow as they can operate quite safely at part openings. A good shutoff can also be achieved if required
Operation: Refer to Figure

When the hand wheel is turned clockwise, the disc is against a seat, stopping the flow. Turning the hand wheel anti-clockwise lifts the disc from its seat and allows flow to continue.

The high pressure is usually on the bottom of the plug, so that the stem, seal, etc. are not under continuous pressure.

Applications are widespread, including domestic water taps

Valves

Valves [Lesson 2]

Specific Types

1. Gate Valves

Purpose: gate valves are used when a tight shut-off is required. They must not be used for throttling (i.e. must be fully off or fully on) as a restricted flow through a gate valve will erode the seat of the wedge disc.

The wedge-shaped disc is moved up to open the valve by turning the wheel anti-clockwise. To shut off the flow, the wheel is turned fully clockwise until the disc is properly seated and covers the opening.

One common type is the rising stem type. In other designs, the wheel is fixed to the stem and rises with it.

The position of the stem/stem and hand wheel indicates whether the valve is open or shut. If the stem is raised, the valve is open.

2. Ball/Plug Valves
Purpose: Ball and plug valves are used to provide a quick, simple shut-off. They are operated by turning the ball or plug through 90º. Ball and plug valves should not be used for throttling, as a restriction in the flow will lead to erosion of the valve.
Operation: Refer to Figure

The ball or plug has an opening through the centre. When this opening is in line with the inlet and outlet ports, flow will be allowed. When the ball or plug is turned through 90; no flow can take place. Good sealing can be obtained, particularly when special sealing rings made of PTFE/Teflon are used.

Some ball and plug valves are lubricated to provide a seal and prevent wear, and should be regularly lubricated with the proper lubricant.

Valves

VALVES [Lesson 1]

Process Valve Types And Applications
Valves are used in both domestic and industrial situations to control the flow of liquids, solids and gases.

The most common and familiar valves are the taps used in the home to control the hot and cold water. In the oil industry, valves are a major element in the control of operations. In general, valves are used for one or more of three main purposes:
1. To control the rate of flow (throttle);
2. To shut off/permit flow (ON/OFF function);
3. To isolate systems and protect products.

There are a wide variety of valve types and designs available from many suppliers in a wide range of materials; the main types and their uses are
- Gate Valve: Used for shut off - ON/OFF function.
- Ball/Plug Valves: Used for shut off - ON/OFF function
- Globe Valves: Used for control of flow and shut off.
- Butterfly Valves: Used for control of flow and shut off.
- Relief Valves: Spring loaded to open at a given pressure, and used to protect systems from over-pressure.
- Check Valves: To allow flow in one direction only.
- Fusible Link Valves/Piston Operated Valves: Quick acting and used for emergency shut off.
- Twin Sea valves: Used when tight shut off required.
- Semi-Needle Valves: Used in conjunction instruments to bleed off part of the flow.
- Ball Check Valves: Used with gauge glasses as safety precaution.

There are other less commonly used types of valves.
The actual construction/design of gate valves, for example, may vary widely depending on its application,the materials used, or the manufacturer’s own special features. The basic principle, however, will be the same.

Valves can be specially made to work at high or low temperatures (cryogenic), or to very high standards for use in explosive atmospheres, or when no leakage is permissible.

Design Features

1. Internal Sealing Systems And Materials
All valves are prone to leakage as it is difficult to obtain a perfect seal, although the use of special seal materials and designs can have very good result If high security is required, use can be made of two valves in series, one to act as the main valve and the second as a back-up should the first fail.

Some valves have bleed holes installed to detect leakage across the seal. If two valves are used together, a bleed hole may be fitted in the pipe between them, which can be opened when the valves are closed to drain any leakage. (Block and Bleed valves)

2. Body/Housing Materials
A wide range of materials is used in valve manufacture, the particular material depending largely on the fluids to be handled. Iron and steel are mainly used for oil/petroleum applications with most valves being made of mild or alloy steel. Brass valves are used for water (as well as cast iron, steel and other alloys).

Stainless steel is used for acids and other corrosive liquids. Bronze is also a commonly used material which can cope with most liquids.

3. External Sealing Systems And Materials
As well as the main seal between valve and disc, wedge, etc. there are other seals required to prevent external leaks. Gaskets or ‘0’ rings are used between surfaces such as flanges, where no relative movement takes place. The main problems occur around the valve stem, which both rotates and, in some cases, moves vertically as well.

Special glands or packings are used which can be compressed by gland nuts to increase sealing. Special materials have to be used in corrosive applications, but an asbestos based fibre is a commonly used packing material with PTFE/Teflon being increasingly common. ‘O’ rings can also be used as shaft seals and are generally made of rubber.

4. Actuation of Valves
Many smaller valves are hand operated if they are accessible. Larger valves require power actuators and inaccessible valves of all types require some form of mechanical or electrical actuator. Pneumatic (compressed air) and hydraulic cylinders and mechanisms are widely used in larger applications. Smaller valves can be operated with solenoids, but larger valves require more complex motors and mechanisms for electrical power operation.

5. Standards of Manufacture
There are many standards to which valves can be made:
• Metric/Imperial dimensions;
• British Standards BS;
• German Standards DIN;
• US Standards ANSI (previously ASA)
• American Petroleum Institution API.

Care must be taken that valves, flanges, etc. and other equipment are compatible, or leakage may occur.
API flanges and other equipment are commonly used in the oil industry. The standards lay down performance requirements as well as dimensions and material. Valves are rated according to the maximum pressure and temperature at which they can safely be used.

6. Quick Closing Valves
Quick closing valves can be installed in pipelines and systems to isolate sections in case of fire, leakage or other emergencies.

A spring is usually used to operate the valve and can be released by a number of methods:
• Fire melts fusible link;
• Remote manual cable;
• Air operated actuating cylinder;
• Electrical solenoid, etc.

If the valve can be installed so that the line pressure will help to close it, this will increase the sealing capability. Swinging check valves are often used as the basis of a quick closing valve, although ball valves, plug valves and butterfly valves are also suitable.

Sizing Of Pipelines

SIZING OF PIPELINES

Oil Pipelines
Pumping a specified quantity of a given oil over a given distance may be achieved by using a large diameter pipe with a small pressure drop, or small diameter pipe with a greater pressure drop.

The first alternative will tend to a higher capital cost with lower running costs. It is necessary to strike an economic balance between these two.

There are no hard and fast rules, which can be laid down for achieving this balance. For instance, a pumping station in a populated area may consist of a simple building, involving the provision of electrically driven pumps, taking power from outside sources and little else. To obtain the same pumping power in remote or undeveloped countries would involve a considerably more complicated and expensive installation. Obviously in this latter case, it is desirable to reduce the number of pumping stations at the cost of using larger diameter piping.

Similarly, the cost of the pipeline will vary considerably, depending upon circumstances. It will be costly in highly industrialised areas, environmentally sensitive areas, offshore or in hostile, mountainous or swamp areas; cheaper in flat, soft but firm, undeveloped terrain.

Gas Pipelines
Sizing problems encountered in gas lines differ considerably from those of oil lines. A simplification results from the negligible weight of the gas as the pressure in the line is virtually independent of the ground elevation on the other hand, the compressibility of gas introduces the complication of the density decreasing and consequently the volume rate of flow increasing in the direction of flow. In an oil line of constant diameter laid on level ground, the pressure decreases uniformly with distance and the velocity stays constant whereas, in a gas line, the velocity increases as the pressure gradient decreases with an exponential, which becomes progressively steeper.

The characteristics of pumps and compressors also determine the site of any pipeline booster stations as well as the initial pipeline conditions which have to be met Pumps need to be sited in positions where they are receiving the crude oil at a pressure greater than the vapour pressure of the crude oil, whereas compressors have to be sited at a location where both the pressure and velocity of the gas are at optimum conditions.