PIPING-KNOWLEDGE

GATE VALVE

A gate valve is generally used to completely shut off fluid flow or, in the fully open position, provide full flow in a pipeline. Thus it is used either in the fully closed or fully open positions. A gate valve consists of a valve body, seat and disc, a spindle, gland, and a wheel for operating the valve.

WORKING OF GATE VALVE:

PARTS OF GATE VALVE

1) Bonnet :

The bonnet of a gate valve contains the moving parts and is attached to the valve body. The bonnet can be removed from the body in order to allow for maintenance and replacing parts.

2) Trim :

The trim of a gate valve contains the functioning pieces of the valve: the stem, the gate, the disc or wedge, and the seat rings.

3) Stem :

The stem of a gate valve is either a rising stem or a non rising stem. The stem is responsible for the proper positioning of the disk. Nonrising stems will almost always have a pointer-type indicator mounted onto the upper end of the stem to indicate valve position. This configuration protects the threads from carrying dirt into the packing because the stem threads are held within the boundary of the valve packing. Rising stems rise out of the flow path when the valve is opened. They can either have a stem that rises through the handwheel or have a stem that is threaded to the bonnet.

4) Seat :

The gate valve is equipped with two seats that carry out and ensure the seal with the gate. Gate valve seats are either integrated with the valve body or in the form of a seat ring. Seat rings are threaded into position or pressed into position and sealed welded to the valve body in the seat ring construction. For greater temperature service, the latter type of construction is preferred. Integral seats are made of the same material as the valve body, whereas pressed-in or threaded-in seats allow for more variation.The seat of a gate valve is either integral with the valve body or in a seat ring type of configuration. The seat ring construction is either threaded on to the body or pressed into position and the seal welded to the valve body. Pressed and welded is recommended for higher temperature applications. Press-in or threaded-in seats permit variation in the seat material verses the material of the body of the valve.

5) Actuator :

Valve actuators open and close the valve in response to a signal or manual manipulation. Most gate valves have manual actuators, such as a handwheel, because they are commonly used in applications where the valve does not need to be opened or closed often or quickly. Since gate valves are not used in throttling applications, the actuator is responsible for fully opening and fully closing the valve.

PRINCIPLE:

The body, seat, gate, stem, bonnet, and actuator are the essential components of a gate valve. The primary mechanism of operation is straightforward. Common gate valves are activated by a threaded stem that connects the actuator, e.g., handwheel or motor, to the gate. The stem is rotated by turning an actuator, which moves the gate up or down via the threads. To fully open or close the valve, it takes more than one 360° rotation. The valve opens by lifting the gate out of the flow path. As the gate is lowered to its closed position, the bore is sealed, and the valve is fully closed.

ADVANTAGES :

Good shutoff features
The body construction is simple, and the production process is more efficient
The length of the structure is rather short
The torque needed to close and open the door is minimal
Gate valves are bidirectional and hence they can be utilized in two directions
Low resistance to flow;
Pressure loss through the valve is minimal

DIS-ADVANTAGES:

They cannot be opened or closed quickly
Gate valves aren’t designed to control the flow
Gate valves often have two sealing surfaces, which makes processing, grinding, and maintenance more challenging
The abrasion and friction of the sealing surface increase while opening and shutting. Furthermore, it is possible to develop abrasions when the temperature is high
In open conditions, they are sensitive to vibration

Gate valves can be divided into two main types:

1) Parallel gate valve

2) wedge-shaped.

Material for Valve Construction

In order to select the proper valve material there are several important criteria to be considered:

1) The composition of the media in contact with all wetted (exposed) parts
2) Service temperatures
3) Operating pressures
4) Effectiveness of coating on materials
5) Material availability and cost
6) Compatibility of materials with injected media.
7) How long the valve will be exposed to the media

There are organizations dedicated to developing and maintaining standards for valves and materials in particular environments. Gate valves are available in many different materials. Valves can be specified by the National Association for Corrosion Engineers (NACE) and the American Petroleum Institute (API) for their ability to handle strong and corrosive media.

1) Valve is designed as per API 600\API 602/ BS5352/ API 603/ API6D/ IS780
2) Valve pressure temperature rating : API B16.34
3) Flange dimension is as per : ASME B16.5, B16.47
4) Face to face dimension is as per : ASME B16.10
5) Butt / Scoket weld end : ANSI B16.25 and B16.11
6) Valve Inspection & testing in accordance with : API 598

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Hydrostatic & Pneumatic Procedure

Hydrostatic testing is most often preferred over pneumatic testing because of safety considerations. Water is the preferred fluid medium for hydrostatic testing. In addition to its relatively incompressible nature, it is also the safest fluid because it is nontoxic and nonflammable and it remains in liquid form down to atmospheric pressure unless heated above the boiling point. Water is readily and economically
available. ASME B31.1, section 137.4.3, specifies water as the test fluid for hydrostatic tests unless otherwise specified. ASME B31.3, section 345.4.1, also specifies water as the test fluid for hydrostatic test unless there is the possibility of freezing or damage from adverse effects of water on the piping or the process for which the piping system is designed. This section provides for other uses of other nontoxic liquids as long as the flash point exceeds 120F (49C). If hydrostatic testing is being considered for a system that was designed for use of a gaseous medium as the process fluid, it must first be determined that the piping system will support the weight of the test liquid or that the piping can be safely supported during the hydrostatic test.

There are some cases where water cannot be employed as a fluid test medium. Traces of water left in the piping may react adversely with the process; for example, in piping for liquid sodium or a cryogenic process. If the test is to be run at temperature conditions which would make freezing likely, antifreeze may be added to the water as long as the antifreeze is not harmful to the piping or the process and the disposal of large amounts of antifreeze is not environmentally unacceptable. If water cannot be employed as the fluid test medium and another liquid is not practical, or if it is not practical to support the weight of a test liquid with piping supports, a pneumatic test may be the next best choice

Checklist for Hydrotest:

● Completed and torqued flanges with no missing bolts or gaskets
● All gravity supports installed
● Proper pipe routing
● Correct valve type and orientation
● Vents and drains installed to allow proper filling and draining

● Proper material type verified using color codes or markings, and heat numbers
recorded if required by the codes
● All required piping stress relief, weld examinations, and welding documentation
completed and acceptable
● Before the test is to be run, make certain, by reviewing the piping against the
P&ID and valve line-up sheet, that
● All equipment not to be tested is disconnected from the test or isolated by closed
valves or testing blinds
● Tagging and lockout of any valves used to isolate the test boundaries is in place
to protect both the testing personnel and any others who may be on site
● All non boundary valves in the test boundary are in the open position
● Expansion joints, if any, have required restraints to protect against damage from
the test pressure
● All springs have travel stops to protect against the weight of the test medium
● All test equipment is checked and all test connections are tight
For gas systems, additional gravity supports may be required temporarily to support the weight of the test liquid. Since requirements vary from project to project, the person responsible for the test will need to make a specific checklist of items for each project before testing can begin.

Hydrostatic Testing Preparation

All joints, including welds and flanges, of the portions of the system to be tested are left uninsulated and exposed for examination during the test. Some insulation may be installed on the straight runs or previously tested piping. If the system is to include jacketed piping, the leak tests should be run before any jacketing is installed.

A flow water pump is substituted for the pressurizing water pump during the filling of the piping. The water source should provide clean chloride-free water. The system is filled from the bottom to facilitate the venting of all air in the portion of the piping system under test. For sloped piping systems, filling should be done against the slope. Vents must be located at all high points in the piping and should be open during the filling stage. Once it is determined the system is completely liquid filled the vents may be closed and a pressurizing water pump connected to the system in place of the flow pump. The pressurizing pump must have a capacity greater than the allowable leakage of the system. Leakage at the packing glands of valves and pumps is permissible by the codes and is necessary to preserve the life of the packing. However, if this leakage is so great that the test pressure cannot be controlled by being trapped, there will be a problem in running the test. It is not very practical to turn the pressurizing pump on and off to maintain the pressure close to the required level. One solution is to temporarily tighten all the packings to a greater compression than is normally used during regular operation of the system. It may also be necessary to tighten flanges, screwed connections, and other mechanical or gland-type joints to eliminate leakage.

Test and Examination Pressures

Test pressure minimum	Test pressure maximum	Test pressure hold time	Examination pressure	Code	Test type
ASME B31.1	Hydrostatic1	1.5 times design	Max allowable test pressure any component or 90 percent of yield	10 minutes	Design pressure
ASME B31.1	Pneumatic	1.2 times design	1.5 times design or max allowable test pressure any component	10 minutes	Lower of 100 psig or design pressure
ASME B31.1	Initial service	Normal operating pressure	Normal operating pressure	10 minutes or time to complete leak examination	Normal operating pressure
ASME B31.3	Hydrostatic	1.5 times design	Not to exceed yield stress	Time to complete leak examination but at least 10 minutes	1.5 times design
ASME B31.3	Pneumatic	1.1 times design	1.1 times design plus the lesser of 50 psi or 10 percent of test pressure	10 minutes	Design pressure
ASME B31.3	Initial service3	Design pressure	Design pressure	Time to complete leak examination	Design pressure
ASME I	Hydrostatic	1.5 times max allowable working pressure4	Not to exceed 90 per- cent yield stress	Not specified, typically 1 hr	Max allow able working pressure4
ASME III Division 1 Subsection NB	Hydrostatic	1.25 times system design pressure5	Not to exceed stress limits of design section NB-3226 or maximum test pressure of any system component5	10 minutes	Greater of design pressure or .75 times test pressure
ASME III Division 1 Subsection NB	Pneumatic	1.2 times system design pressure6	Not to exceed stress limits of design section NB-3226 or maximum test pressure of any system com	10 minutes	Greater of design pressure or .75 times test pressure
ASME III Division 1 Subsection NC	Hydrostatic	1.5 times system design pressure	If minimum test pressure exceeded by 6 percent establish limit by the lower of analysis of all test loadings or maxi- mum test pressure of any component	10 minutes or 15 minutes per inch of design mini- mum wall thickness for pumps and valves	Greater of design pressure or .75 times test pressure
ASME III Division 1 Subsection NC	Pneumatic	1.25 times system design pressure	If minimum test pressure exceeded by 6 percent establish limit by the lower of analysis of all test loadings or maxi mum test pressure of any component	10 minutes	Greater of design pressure or .75 times test pressure
ASME III Division 1 Subsection ND	Hydrostatic	1.5 times system design pressure for completed components, 1.25 times system design pressure for piping systems	If minimum test pressure exceeded by 6 percent establish limit by the lower of analysis of all test loadings or maxi mum test pressure of any component	10 minutes	Greater of design pressure or .75 times test pressure
ASME III Division 1 Subsection ND	Pneumatic	1.25 times system design pressure	If minimum test pressure exceeded by 6 percent establish limit by the lower of analysis of all test loadings or maxi mum test pressure of any component	10 minutes	Greater of design pressure or .75 times test pressure