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Interstitial Space Vent

Assembly/Repairs

General:

There are three basic interstitial vent models in use, 1) the Steel Plug “O” Ring Seal Vent (Fig. 2),  2) the Tapered Plug Seal Vent (Fig. 2), and (3) the Flat Plug Socket Weld Seal Vent (Fig. 2a & 2b).  The sealing process for all models requires the use of Dow-Corning 111 Silicone Grease, Dow-Corning High Vacuum Grease, also a silicone based product, or Permatex Industrial Superlube Lubricant.

For site repair work the Dow-Corning High Vacuum Grease provides the best results.

All ULC Double Wall Tank Standards list the required vent size and the minimum number of vents required, both of which may vary with tank capacity and configuration.

(1) The Steel Plug “O” Ring Seal Vent:

The design for this product is obtained from Figure 13, Page 41 of the ULC-S601-00 Standard for Shop Fabricated Aboveground Horizontal Tanks for Flammable & Combustible Liquids, the figure is duplicated here as Fig.2.

Components:

The components are A) A machined Spigot, generally made from plated or stainless steel, with a machined “O” ring seat, B) A nitrile “O” ring, neoprene has also been used, C) A schedule 80 pipe with a machine “O” ring seat, and D) A spigot retaining chain.

Trouble shooting the Steel Plug “O” Ring Seal Vent:

Release the remaining vacuum on the interstitial space by opening valve C (Fig. 1).

Remove Machined Spigot A  (Fig. 2) clean and examine machined “O” ring seat. Where it is found that the seat is covered with surface rust, sand the seat using fine emery cloth. After cleaning apply a layer of Dow-Corning High Vacuum Grease over the entire surface of the seat.

With the plug removed perform a simple scent test at the open interstitial vent nipple to determine if there is product in the interstitial space. The odor of gasoline or diesel fuel at the open vent indicates that the tank may be leaking and further precision leak detection testing is required.

Remove the “O” ring, the “O” ring shall be replaced every time the vent system is maintained.


Clean and examine the machined “O” ring seat on the schedule 80 pipe. Where it is found that the seat is covered in surface, sand the seat using fine emery cloth. After cleaning apply a layer of Dow Corning Silicone Grease over the entire surface of the seat.

Replace the “O” ring so that it seats in the applied grease seal, replace the machined spigot on the “O” ring and re-establish vacuum on the interstitial space.

2) The Tapered Plug Seal Vent:

The Tapered Plug Seal Vent (Fig. 2) consists of two components, E) a custom moulded urethane elastomer tapered plug and F) a threaded one end or plain Sch 40 pipe nipple with a machined tapered seal that matches the taper on tapered plug.

Trouble Shooting the Tapered Plug Seal Vent:

Release the remaining vacuum on the interstitial space by opening valve C (Fig. 1).  Remove the tapered plug.

With the plug removed perform a simple scent test at the open interstitial vent nipple to determine if there is product in the interstitial space. The odor of gasoline or diesel fuel at the open vent indicates that the tank may be leaking and further precision leak detection testing is required.

The availability of the tapered plug is very limited.  Clean and examine the plug, the contact area of the tapered plug to the metal seat will be deformed and may be compressed to the point of being rigid, where this is the case the plug must be replaced.

Where a replacement plug cannot be found, the venting system should be replaced with the Flat Plug Socket Weld Seal Vent (Fig. 2b) using the following procedure:
a) Where the machined Schedule 40 vent pipe nipple has been threaded, it can be replaced with a socket weld fitting that has been welded to an equal sized Schedule 40 nipple that is threaded at one end; or
b) Where the machined Schedule 40 vent pipe nipple has been directly welded to the tank, an equal sized socket weld coupling may be welded to the existing machined nipple, or the existing nipple can be threaded and adapted by the use of a Schedule 40 half coupling that has been welded to a socket weld coupling.

Note: When performing hotwork, such as welding, in a hazardous area it is necessary to take all the safety precautions under the Health and Safety Act and Department of Labour Regulations.


Where it is found that the tapered plug is re-useable or a new plug is available, the machined seat of the Schedule 40 vent nipple shall be cleaned, surface rust shall be removed by using fine emery cloth sandpaper and the machined seat covered with a layer of Dow-Corning High Vacuum Grease.

Prior to placing the tapered plug onto the machined nipple, the seating area of the tapered plug shall be covered with a layer of Dow-Corning High Vacuum Grease. Place the treated plug on the vent nipple, apply a vacuum to the interstitial space of the double wall tank.       

3) The Flat Plug Socket Weld Seal Vent:

The Flat Plug Socket Weld Seal Vent (Fig. 2a & 2b) consists of two components, A) a flat urethane elastomer plug, and B) a 3000lb socket weld elbow or coupling.

Trouble Shooting the Flat Plug Socket Weld Seal Vent:

Release the remaining vacuum on the interstitial space by opening valve C (Fig. 1).  Remove the urethane elastomer plug.

Using a self tapping 1.5” sheet metal screw, thread the screw into the centre of the plug A (Fig. 2b) to a depth of 1”, using the screw as a handle pull the plug out of the socket weld fitting. Remove the screw and discard the used plug.

With the plug A removed perform a simple scent test at the open interstitial vent socket weld fitting to determine if there is product in the interstitial space. The odor of gasoline or diesel fuel at the open vent indicates that the tank may be leaking and further precision leak detection testing is required.

Clean the internal sides and shoulder of the socket weld fitting B (Fig. 2b), removing the existing grease. Where surface rust is found on the internal sides or shoulder of the socket weld fitting it can be removed by using fine emery cloth sandpaper. After cleaning, apply a layer of Dow-Corning High Vacuum Grease to the internal sides and shoulder of the socket weld fitting (Fig. 2b), the shoulder application should result in filling the 900 shoulder to internal side angle with a bead of grease.

Apply a layer of Dow-Corning High Vacuum Grease to the side of the replacement plug prior to inserting the plug into the socket weld fitting. The replacement plug can then be inserted into the socket weld fitting, do not bottom out the plug by hand, the plug will bottom out when vacuum is applied to the interstitial space.

With the plug inserted into the socket weld fitting apply a vacuum to the interstitial space, the plug should be pulled into the socket weld fitting by the applied vacuum and a bead of grease appear at the exterior plug to socket weld shoulder joint.  


Vacuum Application Methods:

There are two basic models of vacuum pump used, 1) the standard vacuum pump powered by an electric motor available at refrigeration repair parts distribution companies; and 2) the venturi type educter which requires a compressed air source (Fig. 3). The selected vacuum pump must be capable of establishing a minimum vacuum of 25” Hg.  It is advisable to select a high-speed vacuum pump to reduce the time required to establish vacuum.

The ULC Standards require that a minimum vacuum of 18”Hg. (-63kPa), since vacuum gauges are generally marked in 5 or 10 units, it is suggested that the vacuum used for repair purposes be either 20”Hg. or –70kPa. The same level of vacuum should be used for each repair, it eliminates interpretational error.  

It will take several days to determine if the repair procedures were satisfactory, the larger the capacity of the tank the longer the period required. It is a general requirement of the ULC Double Wall Tank Standards that “the vacuum shall be recorded daily for over a five-day period”, a continuous decrease in vacuum gauge reading over the five-day period indicating that further remedial action is required.


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