Barney's Blog no 4 - Finding Pipe Joints in Cast Iron Gas Pipes

Many towns and cities had their gas utility system installed more than 70 or 80 years ago and will almost certainly use cast iron pipes for the urban low pressure gas distribution system.


Gas distribution systems were also using "town gas" at the time. This was gas derived from coal, and required a manufacturing plant and gasholders for storage and pressure maintenance. At the time natural gas was not available.

Due to its manufacturing process, town gas had a high water vapour content, and the water vapour condensed on the inside of the underground pipes, and gradually filled the pipe. Gas distribution companies were aware of the problem, so the pipe system was designed with a gradient, and the condensation ran down to a collection sump. From the bottom of the sump there was a pipe to the surface, and this allowed the condensate to be blown out by the gas pressure and collected so that the pipe did not fill with condensate.
The cast iron pipe was laid in sections of generally uniform lengths and the joints between sections were sealed in various ways depending on engineers' preference. One method was to use molten lead, poured through a hole drilled in the top of the bell part of the pipe socket, When it solidified the pipe was almost soldered to the next pipe. Various types of fibrous materials were also used, together with a sealing caulk that bound all the structure together.
Generally as pressure was low either of the sealing methods was acceptable.

Town gas manufacture was polluting, and town gas manufacturing plants tended to be near to urban areas, to reduce pipe length and pressure losses. Natural gas, when it became available was used, it was less polluting and at higher pressure transported from outside the urban area to pressure reduction valves, and into the gas mains, replacing town gas.
As natural gas is vapour free, one of the new problems facing gas distribution engineer was that the fibrous material used for pipe jointing, dried out. Additives are used with the natural gas to keep the pipe joints damp, but there were issues with gas leaks from gas mains, and a need to find the leaking joints and fix them.

The gas distribution companies and their contractors devised several different methods to seal leaking joints. One method involved exposing the circumference of the whole pipe, drilling the bell housing of the joint and first squirting in sealant under pressure. Next the whole pipe joint is wrapped in a circumferential sleeve and a sealant is pumped into the sleeve until the space between the sleeve and the bell of the pipe joint is filled. Finally wait for the sealant to solidify before backfilling the excavation. This method required a relatively large excavation.

Another method was less invasive, and could be performed by "keyhole excavations". It needed accurate location for the joint, so positioning of the excavation and drilling device to squirt in the sealant into the annular aperture around the bell joint was crucial to this much quicker method.

Several methods were available to locate the pipe joints. Easiest is to use a magnetometer type metal detector, such as the FM880B or Schonsted magnetometers, walk the line of the pipe after locating it and detect the greater response of the magnetometer at the pipe joints. The method is easy enough and its results are also confirmed if the operator knows the pipe lengths used, as the maximum response will be obtained over the joints.
However, if the street contains reinforcing bars and other iron or steel utility lines, then it is possible for the operator to become confused; so another method is also needed.

 

Using transmitting sonde for pipe joint location

Two and a half decades ago one of Electrolocation's underground utility surveyors, Mr Trevor Fern, was having difficulty locating a sewer pipe using a drain sonde. Drain sondes are easy to locate in non metallic pipes, but the transmitted signal is no longer detectable if the sonde is within a metal pipe. This is exactly what happened. Mr Fern did not give up easily, so told his colleague to push harder on the drain rods, and walked forwards above the pipe route. Quite by chance the drain sonde signal returned, but only briefly as his colleague pushed sonde forward. Mr Fern got his colleague to push and pull until he achieved the best result, and then push forward again. The signal was lost, but about 3M further Mr Fern found it again. Eventually the pipe returned to usual non metallic construction and was easy to locate. So the "Fern Effect" was discovered as a method to find pipe joints.

 

Inserting sonde into live gas distribution mains

Adapting this application to find pipe joints in cast iron gas mains took a few more years, as the most useful requirement for gas industry was to insert the drain sonde into a live gas pipe under pressure.

The WASK Company manufactured equipment for live under-pressure drilling of iron gas mains, usually used for installing new gas services from the main.

The under-pressure drilling system uses a saddle with adaptors for different pipe diameters and seal between it and the pipe, all chained on to the pipe. Mounted on top of the saddle is a thin plate valve assembly, used to seal the hole drilled in the pipe and avoid gas escaping.
On top of the valve a further adaptor to take a drilling station or other equipment for "bagging off" (blocking) the gas pipe.
Usually all operations are carried out vertically, but in order to push a flexible fibreglass flexrod with sonde on the end along the pipe, the insertion tube is manufactured for 45 degree insertion, and rotatable through 180 degrees so that the sonde can be used in both directions from one position.
The first step in the process is to drill a hole in the pipe wall; the drilling head is fitted with a pilot drill and hole cutter, mounted on the plate valve, valve is opened and the hole drilled. The circular plug taken from the pipe remains in the drilling head. Drilling head withdrawn and plate valve closed. Drilling head removed from plate valve.
The insertion tube assembly is mounted on top of the plate valve.
The insertion tube has a seal at its top end through which the flexrod is pushed and the sonde is mounted inside when its battery is inserted, switched on and checked. The sonde is fitted into the insertion tube, and sealed off.
Next the plate valve is opened slightly to fill the insertion tube with gas, and the insertion tube itself is purged of air so no gas and air mixture is present.
Now the plate valve can be opened fully and the sonde inserted into the pipe.

 

Pinpointing pipe joints in cast iron gas pipes

Two operators needed. One has the locator, held in sonde orientation along the line. The other is responsible to propel the sonde /flexrod through the insertion tube and along the line. Generally the flexrod operator will push about 1M and the locator operator walks 1M until signal is detected, and then rod man and locator man adjust rod position until, strongest signal is detected. At this point both sonde and locator are pinpointing the joint. Joint is marked and the flexrod operator and locator operator continue step by step until distance is covered and all joints located.
Flexrod is then pulled back and reeled in until sonde is back in the head of the insertion tube.
Plate valve is then closed. At this point there is a choice if the operators want to locate joints along the opposite direction of the pipe, or are finished.
Finally the drilling head is remounted fitted with a hole tap, to tap a thread into rim of the hole through the pipe.
Once tapped, a threaded plug can be screwed into the hole sealing it, so at this point the drill kit can be dismounted, along with the valve, adaptors seals and saddle, as the task has been completed.

Email barney@mansls.co.uk for anymore info on this subject.