Form no : 08/02

SCHOOL OF MARITIME STUDIES Date : 15/02/2012

VELS UNIVERSITY Issue no : 02/12
THALAMBUR Rev. no : 00

GATE VALVE

AIM:

To study the application of gate valves on board merchant ships worldwide, their classification, their design,
cautions in operation, maintenance procedures, and the testing of valves after overhauling.

THEORY

The gate valves are so called because the gate-shaped wedge inside is the most used, as well as the most abused,
type of valve. Though designed to isolate or connect fluids, they are sometimes wrongly used for flow control.
The gate valve should not be used for controlling the fluid flow or for throttling the flow as it can damage the
valve due to the high velocity of fluid when the valve is only slightly open. They are normally used in
applications where a minimum restriction to the flow of fluid is a design requirement and only an On-Off type
of control is required.

CLASSIFICATION OF GATE VALVE

Gate valves can be classified on the following basis:

1. Stem: Whether the stem can rise or is not rising.
2. Shape of gate: the gate can be wedge shaped, knife shaped, or of parallel construction.
3. Material
4. Working Pressure
5. The distance between the flanges
6. Ends: flanged or threaded

COMPARISON BETWEEN GATE VALVE AND GLOBE VALVE

1. Gate valves are used for On-Off control whereas Globe valves in addition can also be used for the flow
regulation.
2. Gate valves offer very little resistance to fluid flow in fully open position and also have small pressure drop
across the valve. Globe valves on the other hand have a high pressure drop even in fully open conditions and
offer substantial resistance to fluid flow.
3. Gate valves of the same size are cheaper than globe valves.
4. Gate valves because of their design have very little fluid trapping in the line but the globe valves have a
larger amount due to the direction of flow.
5. Gate valves are unidirectional and can be put around in any way. The globe valves are not.

The gate valve must be protected in the following ways:

1. The valve must be either fully open or fully shut and no intermediate position is allowed.
2. The valve should not be over-tightened with the help of long pipes or wheel spanners as distortion of the
gate can occur.
 Form no : 08/02
SCHOOL OF MARITIME STUDIES Date : 15/02/2012
VELS UNIVERSITY Issue no : 02/12
THALAMBUR Rev. no : 00

3. If the valve is getting stuck, it might be due to the deposition of silt and mud in the guides. Slowly opening
and closing against the fluid flow many times could clear away the deposits.
4. The valve should be fully closed with the hand wheel tight. However when the valve is fully opened, close it
a quarter turn to indicate to the other watch keeper that it is open. This same exercise was discussed in the
case of globe valves.
5. The gate valve can be used in either direction. That means that the suction and the discharge can be
reversed.
6. The gate valve should not be installed upside down horizontally. As it would encourage the deposit of
sediments in the guide ways. However they can be used for vertical duties, but this also should not be
encouraged.

MAINTENANCE:

The gate valves suffer from one common problem after long service and that is the valve becoming tight due to
the deposition of silt on the guide ways. This type of complaint is common and is rectified by opening and
cleaning the valve. But in case the valve has been misused and has been used for throttling of the flow,
generally wire drawing or erosion cutting of the wedge takes place which must be corrected by replacing the
valve, unless we have a spare gate from another old valve. The procedure of overhauling the gate valve is as
follows:

1. Open the wheel nut and remove the hand wheel.

2. Open the gland bolts and slacken the gland flange and the gland bush.
3. Open the bonnet bolts and remove the bonnet along with the bonnet and the valve wedge.
4. Unscrew the valve stem from beneath the bonnet.
5. Clean all the parts by buffing and then blowing with compressed air.
6. Examine all the parts for damage and wear. Any marks on the threads must be repaired by dressing the
threads.
7. The grooves between the wedges seats must be removed by a scraper but be careful not to damage the seats.
8. In case the gate or the seats are damaged and it is removable the same can be changed.
9. After the cleaning the same can be assembled in the reverse order.

TESTING OF THE GATE VALVES

The testing of the valves can be done in a similar way as the globe valves. In brief the valve should be filled up
with water and then air should be given from the other side. The presence of air bubbles would indicate bad
sealing and must be rectified.

PRESSURE TESTING OF VALVES

 After the valve has been overhauled, it must be tested properly before use.
 The testing procedure is such that the valve is to be filled with water above the valve disc (after the valve is
closed), and in first step, the engineer must check whether any water is leaking to the other side.
 Form no : 08/02
SCHOOL OF MARITIME STUDIES Date : 15/02/2012
VELS UNIVERSITY Issue no : 02/12
THALAMBUR Rev. no : 00

 If any leakage is there in the first test, then the valve and the valve seat have to be re-lapped and then re-
tested. If the valve is now satisfactory and there is no leakage, then the second step of the testing should be
done.
 An appropriate flange has to be put and compressed air has to be put from the lower flange, below the valve
seat at a pressure equal or more than the service pressure. If no bubbles are observed on the water at the top,
then the valve is considered to be holding and it has been correctly overhauled.
 The other method of testing is to put a compressed air connection, with a regulating valve and a pressure
gauge to one side of the valve, with good gaskets and tighten it. Thereafter the valve should be closed tight,
and compressed air at a pressure equal or more is put in and the regulating valve closed. Next the drop in
pressure is observed. If there is no pressure drop for five minutes, then the valve is holding and the test is
successful.
 After the testing has been successful, then the valve should be put in a proper location, with a tag stating that
the valve has been tested, the date of testing, and the name of the testing engineer.

RESULT:

Thus the application of gate valves on board merchant ships worldwide, their classification, their design,
cautions in operation, maintenance procedures, and the testing of valves after overhauling is studied.
 Form no : 08/02
SCHOOL OF MARITIME STUDIES Date : 15/02/2012
VELS UNIVERSITY Issue no : 02/12
THALAMBUR Rev. no : 00

OVERHAULING OF BUTTERFLY VALVE

AIM:

To study the constructional details and overhauling of a butterfly valve.

TOOLS REQUIRED:

Set of spanners, pipe wrench, hole punches, ball pean hammer, screw driver, chisel.

CONSTRUCTIONAL DETAILS:

A butterfly valve is a valve which can be used for isolating or regulating flow. The closing mechanism
takes the form of a disk. Operation is similar to that of a ball valve, which allows for quick shut off. Butterfly
valves are generally favored because they are lower in cost to other valve designs as well as being lighter in
weight, meaning less support is required. The disc is positioned in the center of the pipe, passing through the
disc is a rod connected to an actuator on the outside of the valve. Rotating the actuator turns the disc either
parallel or perpendicular to the flow. Unlike a ball valve, the disc is always present within the flow, therefore
a pressure drop is always induced in the flow, regardless of valve position.

PROCEDURE:

1. Precaution is to be taken to isolate the pipeline

and de-pressurize the system in which the valve is
fitted.

2. The disc and the renewable elastomer ring are

cleaned thoroughly.

3. Check the condition of the disc in the valve.

4. If the disc is damaged, change the disc

5. Check the condition of the renewable

elastomer ring.

6. If the ring is damaged. Change the ring.

7. The valve is inserted in the pipeline and

checked for goodness of valve.

RESULT:
 Form no : 08/02
SCHOOL OF MARITIME STUDIES Date : 15/02/2012
VELS UNIVERSITY Issue no : 02/12
THALAMBUR Rev. no : 00

Thus the gate valve was overhauled.