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Vaccum Gauge



At sometime during the life of a ULC listed double wall vacuum monitored tank it is expected that the vent and vacuum gauge assembly systems will fail due to mechanical problems. The following information supplies the corrective measures necessary to return these systems to working order. All threaded pipe connections, including those to electrical conduit, will require the use of a ULC listed pipe thread sealant. Dow-Corning High Vacuum Grease will also be required to perform the repair processes.


Note: In order to maintain the ULC listing on the Double Wall tank as a monitored system it is necessary that when components are replaced, they either be purchased from the original ULC listed tank manufacturer or from a ULC listed component supplier.

Vacuum Gauge Assemblies:

Vacuum gauge assemblies come in varying layouts, they however consist of 6 basic components (Fig. 1), A – Vacuum Gauge, generally glycerin filled with either centre back or bottom outlets depending on the monitor position; B – Plated Steel or Stainless Pipe Cross or Tee the choice being dependant on whether a vacuum switch is required;   C – Brass or Stainless Steel Needle Valve; D – Vacuum Switch, generally a normally closed system that is open under vacuum, also available in a normally open configuration;  E  - 1” NPT Plated Steel Pipe Coupling to provide for protection of the vacuum switch and a means of attaching conduit; F – 1” NPT male x 1/8” NPT female x ¼” NPT Vacuum Switch Adapter Plated Steel; G  - ¼” male NPT Tank Connection, Nylon hose is used for underground tank installation, plated steel nipples for aboveground tank applications.
A – Vacuum Gauge

– As a minimum it is advisable to use a stainless steel, glycerin filled vacuum gauge rated to –400C atmospheric exposure.
Glycerin filled vacuum gauges are generally equipped with a plug on top of the gauge that seals the glycerin in during shipping and handling, in order to provide an accurate reading the glycerin must be exposed to atmospheric pressure, it is therefore necessary to either remove the plug or snip the tip off the top of the plug to acquire a correct vacuum reading.

Installation of the Vacuum Gauge Assembly system requires that the gauge always be in the upright position to prevent leakage of the glycerin.  

Troubleshooting the Vacuum Gauge:

Replace the vacuum gauge when, 1. The glycerin level in the gauge is depleted, this generally signifies that there is a leak in the bourbon tube that is internal to the gauge; or 2. Where, with the valve C in the open position, the gauge does not return to 0; or 3. The vacuum gauge pointer does not move when vacuum is applied to the system.

B – Pipe Cross or Tee

The pipe cross or tee, the tee being used where a vacuum switch is not required, is generally constructed of plated steel. It is advisable to use a dry tight thread design with a ULC listed pipe dope when making threaded connections to the cross or tee.

Troubleshooting the Pipe Cross or Tee

Where the Vacuum Gauge Assembly is located below grade level in a sump, the steel pipe cross or tee are subject to galvanic corrosion because of the presence of moisture and the attachment of a dissimilar metal, the brass needle valve C. It is often found that the cross or tee is so badly corroded that the male threads of the needle valve are exposed. The plated cross or tee when below grade should be replaced with a stainless steel unit.

C – Needle Valve

The needle valves used are commonly brass needle valves. Needle valves are preferable because they can be snugged down with a wrench and not easily opened by hand. A metal to metal needle valve seal is preferred, brass being softer than stainless provides a better seal.  Ball valves should not be used, since they include gasketed seats that tend to deteriorate over time.

Troubleshooting the Needle Valve:

The correct position of a needle valve in a vacuum system requires that the sealing  seat and the needle be visible through the open end of the valve not connected to the tank (Fig. 1a), this removes the possibility of a leaking valve packing as a cause of failure.

Every time the valve is opened for maintenance remove the needle (N), coat the tapered end of the needle with a thin layer of Dow-Corning High Vacuum Grease and replace needle in valve.

Always snug needle down into the sealing position with a wrench after hand tightening.

D – Vacuum Switch

The vacuum switch is an automotive switch that is normally closed, it opens under vacuum, and constructed of plastic and plated steel. Normally open vacuum switches that close under vacuum are uncommon but also in use.
Note: Other models of vacuum switch may include totally enclosed systems the maintenance and repair of these systems will require the original manufacturer’s instruction.

Troubleshooting the Vacuum Switch:

Where the vacuum switch is connected to a remote monitor alarm panel and the panel is in alarm, access the Vacuum Gauge Assembly and check the vacuum gauge to determine if there is still a vacuum on the interstitial space. Where there is a vacuum on the interstitial and the monitor is in alarm, remove the electrical connection to the vacuum switch, use a multimeter to determine if the vacuum switch is closed or open. Where the panel is in alarm and the vacuum switch is open, the vacuum switch is probably being shorted out by moisture, replace the switch.

In accessing the switch should water be found in the 1” coupling, check the conduit connections for leakage, water leakage may occur in the conduit where the installing electrician did not use pipe dope with the threaded electrical connections.

Check the electrical connections to the vacuum switch terminals, they should be made with plastic coated terminal connections to prevent shorting to the 1” coupling.

E & F – 1”NPT Plated Coupling and Vacuum Switch Adapter

The plated coupling and vacuum switch adapter are threaded NPT connections, the adapter is custom made.

Troubleshooting 1” NPT Coupling and Vacuum Switch Adapter

Inspect coupling attachment to electrical conduit, the connections require the use of pipe dope to eliminate any water leakage at the coupling to electrical conduit connection.

G – Nylon Tube or Steel Nipple Connection

The nylon tube connection consists of 48” of ¼” O.D. black nylon tube with a minimum .062” wall. The tube uses either brass or stainless steel compression fittings, the assembly of the compression fittings to the tube requires the use of a male insert inside of the tube. This connection is required where the tank is an underground tank and the vacuum gauge is intended to be located at grade level.

Where the tank is an aboveground tank the nylon vacuum tube assembly is replaced by a ¼” NPT Plated Steel or Stainless Steel Nipple.

Troubleshooting the Nylon Tube Connection:

Remove the nylon tube from the tank by loosening the compression fitting nut at the tank end of the tube, do not remove the part of the compression fitting that is threaded into the tank unless proven to be absolutely necessary. Examine the nylon tube for cracks at the tube to brass fitting juncture, cracks in the nylon tube are generally the result of someone accidentally stepping on the tube fitting attached to the tank at time of tank installation.
Inspect the nylon tube ends to ensure that a male insert has been used in the end of the tube during the assembly process. Inspect the compression fitting nut for hair line cracks which can result in leakage.  

Where the tube is found to be without cracks re-install the tube by coating the tube area immediately below the compression ring with a layer of Dow-Corning High vacuum grease, this will permit the compression ring to re-seat itself when the tightening process is complete. Do not over tighten the compression nut since this can result in a cracked compression nut and loss of vacuum over time.

Troubleshooting the Steel Nipple Connection:

When installed in an aboveground tank the vacuum gauge is generally placed in an upright position by the tank manufacturer. Where it is found that the vacuum gauge is not perfectly in the upright position, the connection of the Vacuum Gauge Assembly to the tank should be checked to ensure that the nipple used to attach the monitor to the tank has not been accidentally backed off during the installation process for the electrical conduit.

Removal of the steel nipple from the tank will require that the nipple be re-attached to the vacuum gauge assembly prior to assembly to the tank, a cap be placed on the nipple and a vacuum of 20” Hg drawn on the assembly. The assembly should then be left over night under vacuum, there should be no loss of vacuum.

An alternate solution is to always have a suitable vacuum gauge assembly under vacuum and replace the entire faulty unit.  This eliminates all the connections systems except the final connection to the tank.   

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