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Crane Pipe Fittings and Valves

Guidance notes for the assembly of Crane Pipe Fittings and Crane Valves having taper thread connections

 

Thread condition and cleanliness

Threads should be clean and free of damage or a pressure tight seal may not be achieved. Any thread protectors should not be removed until the items are ready for assembly.

External threads are the most vulnerable to damage. The threads should be wire brushed if dirty or rusty, and then inspected for damage. Running a die nut down the thread may rectify minor damage. Pipe fittings should be replaced where there is significant thread damage. It may be possible to reclaim pipe with damaged threads where a sufficient length of pipe is available to allow the damaged thread to be removed and the thread recut, otherwise the pipe should be replaced.

Internal threads are more difficult to clean, particularly small ones, but less prone to damage. The threads should be cleaned if dirty or rusty or if other debris is present and then inspected for damage. It may be possible to remove any dirt or other debris using an airline or, for larger threads, a wire brush. Inserting a tap may rectify any minor damage and may remove any rust. Any pipe fitting or valve in which the thread is significantly damaged or cannot be satisfactorily cleaned should be replaced.

 

Cutting threads on pipe.

It is important that the finished thread has the correct diameter, length and taper and is free from damage. The use of proprietary thread cutting tools/equipment and thread gauges will help to ensure that a satisfactory thread is achieved. Where thread gauges are not available, then the thread diameter may be determined by a trial assembly with the mating pipe fitting or valve but this practice is not recommended where large numbers of threads are involved or where the mating pipe fittings or valves are not available.

BS 21, ISO 7 and BS EN 102261 Part 3 give guidance on the use of thread gauges and are available from BSI Customer Service (Tel.  020 8996 9001).

The names of thread gauge manufacturers can be obtained from

GTMA (Tel. 01844 274222)

Adjustable dies should be set accurately to ensure the thread is of the correct diameter.  Generous application of a high pressure cutting lubricant should be used. The spray type has the advantage of clearing the swarf (cuttings) from the die. Clearing the swarf is more difficult on smaller sizes of die which need more frequent replacement because of this problem.

The clutch should be disengaged as soon as the end of the pipe with the cut thread reaches the outer face of the die nut. If disengagement is too soon, the diameter of the thread may be too large and may result in insufficient thread engagement with the mating pipe fitting or valve. If disengagement is too late the diameter of the thread may be too small and in extreme cases a parallel length of thread may be produced on the end of the pipe. This may result in the small end of the pipe making unwanted contact with some part of the pipe fitting or valve before the threads themselves are in pressure tight contact and may prevent a pressure tight seal. Where this unwanted contact is with a valve seat, distortion of the valve seating may occur, a condition known as 'pipe ending'.

 

Thread sealant

Using a suitable thread sealant will assist with achieving a pressure tight seal and is particularly important where the application involves high pressures or gases. The choice of sealant should be guided by the application and any applicable regulations. The use of hemp is not necessary.

The sealant used on assemblies intended for potable water applications should conform to the applicable U K Regulations. Details of sealants approved for use on potable water applications can be obtained from WRc (Tel. 01793 865000)

Generally, the sealant should be applied to the external thread only, particularly where valves are involved, to prevent excess sealant being carried onto the valve seat.

Where PTFE tape is used it should be wound on in a clockwise direction, looking from the small end of the thread. Starting at the large end of the thread will help keep the tape in place during assembly. The extent to which each turn of tape over laps the previous turn will depend upon the tape thickness and the size of the thread. It is usual to apply an overlap of at least half the tape width.

Anaerobic sealants may require special application techniques and the manufacturer's instructions should be followed. Heavy grade tape should be used on sizes above 1inch.

Various grades of PTFE tape are specified in BS 7786 and BS EN751-3. Other types of sealant are specified in BS EN 751-1 and 751-2. All of these standards are available from BSI Customer Services (Tel.  020 8996 9001).

 

Assembly

If using a vice, it is best to hold the pipe in the vice, clear of the thread, and assemble the pipe fitting or valve to the pipe. Pipe fittings and valves are prone to damage by distortion if held in a vice.

Always use the correct size of wrench for the size of pipe. As a rough guide, the length of wrench should be about 12 times the pipe diameter. It is better to use parallel jaw type wrenches or spanners when tightening a valve onto the pipe. Using the type of wrench which self tightens when the load is applied (for example Stilsons) may cause damage if used on a valve.

Valves should always be tightened with the spanner on that end of the valve joining the pipe. Otherwise, with valves such as two piece ball valves, tightening from the opposite end of the valve may accidentally disturb the valve jointing and cause a leak. The assembly should first be assembled hand tight (although a spanner or wrench may be used to reach the hand tight condition). Next the assembly is tightened to give a pressure tight seal. The level of effort applied by the fitter should be guided by the number of turns beyond the hand tight condition. These extra turns are known as the 'wrenching allowance' and are given in the table below. It is unlikely that it will be necessary to apply the full wrenching allowance and the figures in the table should be regarded as a maximum.

 

Thread size

Wrenching allowance

Thread size

Wrenching allowance

Turns

mm

Turns

mm

1/8

1.1/2

1.4

1.1/2

1.1/2

3.5

1/4

1.1/2

2.0

2

2

4.6

3/8

1.1/2

2.0

2.1/2

2.1/2

5.8

1/2

1.1/2

2.7

3

2.1/2

5.8

3/4

1.1/2

2.7

4

3

6.9

1

1.1/2

3.5

5

3.1/2

8.1

1.1/4

1.1/2

3.5

6

3.1/2

8.1

 

Extra care is needed where it is necessary to stop tightening a pipe fitting or a valve to give a particular position, such as having the handwheel on a valve in an upright position, for example. In such cases it will be necessary to decide whether or not to stop tightening when the required position is reached or whether to continue for a further turn. Generally, continuing for a further turn is to be preferred unless there is some doubt that a further turn is possible. If the effort required to complete the extra turn becomes excessive then it will not be possible to reverse the tightening more than a quarter turn or so without the risk of losing a pressure tight seal. Consideration should be given to anaerobic sealants which are more tolerant in such situations but may have unacceptable temperature limitations.

It is not practical to use the number of exposed threads as a guide as to whether or not the pipe has been tightened sufficiently. Leaving aside the so-called wash out threads (threads which are not fully formed at the root) which will always remain exposed, the number of exposed threads will depend not only on the wrenching effort applied but also the diameter of the internal thread and the material and physical size of the pipe fitting or valve.

These notes are intended for guidance only.  Although Crane will accept responsibility for any valves or fittings which are shown to have been incorrectly manufactured, they can not accept responsibility for any leaks or failures resulting from the fitting practices used.