GB2234592A

GB2234592A - Tank contents monitoring unit

Info

Publication number: GB2234592A
Application number: GB9016509A
Authority: GB
Inventors: Stanley Morgan Maude
Original assignee: Tankmaster Ltd
Current assignee: Tankmaster Ltd
Priority date: 1989-08-01
Filing date: 1990-07-27
Publication date: 1991-02-06
Also published as: GB9016509D0

Abstract

The depth of liquid in a tank is monitored by a differential pressure sensor 30 which compares pressure above the liquid with that at a known depth in the liquid and transmits the electrical sensor output to a remote operation room. A monitoring unit, including sensor 30 is mounted in a hole in the tank by screw thread 14. Pressure probe 11 extends to a known depth in the tank and is connected via head fitting 22 and tube 29 to one side of sensor 30, the other side of which is connected via chamber 15 and a passage in mount 12 annular surrounding probe 11 to the top of the tank. To determine the density of the liquid, a second pressure probe may be provided. <IMAGE>

Description

TANK PRESSURE MONITORING UNIT The present invention relates to the monitoring of the contents of liquid in a tank and is particularly concerned with a tank monitoring unit suitable for use with toxic materials.

At present there are two methods of taking the pressure from different vertical positions in a tank which are necessary for the calculation of depth and specific gravity of the liquid in the tank. The first method is to use probes which are fitted into the tank.

The gas pressures from the probes and that from the gas above the liquid in the tank are fed via respective return lines to a remote operations room where they are input to the appropriate side of a differential pressure transducer. The calculation of the contents data is described in our European patent specification No 0048589. The problem with this arrangement is that there is a risk that vapour from the tank leaks into the return lines and thus in some situations into the operations room. With toxic vapour this is obviously undesirable. It is not satisfactory to solve this problem by means of an additional valve in the return line as it would affect the pressure values.

The alternative method of taking pressure readings from a tank is to use a sealed transmitter. This is satisfactory in some situations but it is not possible to fit such transmitters to existing underground tanks.

It is also undesirable to use this method on existing liquid storage tanks as it is very seldom convenient to empty the tank to make it possible.

According to the present invention there is provided a method of monitoring the liquid contents of a tank comprising delivering a first pressure value representing tank liquid pressure at a known depth to one side of a differential pressure transducer and delivering a second pressure value representing tank pressure above the liquid to the other side of the differential pressure transducer wherein the differential pressure transducer is located at the site of the tank and the method includes the step of delivering an electrical signal representing the difference between the first and second pressure values from the tank location to a remote operations location.

The invention also provides a tank pressure monitoring unit for fitting to the top of a tank for carrying out a method according to the preceding paragraph; the unit comprising a pressure sensing probe for delivering a first pressure at a first outlet representing the pressure at a known depth in the tank, and a conduit for delivering a second pressure at a second outlet representing pressure above the liquid in the tank, wherein the unit includes a differential pressure transducer; the first and second outlets are fitted to respective sides of the differential pressure transducer; and the output from the unit, in use, is in the form of an electrical signal from the pressure transducer.

Advantageously the unit is adapted so that the differential pressure transducer is arranged at a sufficient angle from the horizontal with the lower end attached to the second outlet so that condensation of vapour from the tank does not accumulate at the transducer terminal.

In one embodiment the unit comprises a probe mount having a conduit therethrough to receive the probe and a first coupling adapted to make a sealed connection with the tank while allowing the probe to pass through the first coupling and through the hole in the tank, the mount including a second coupling for making a sealed connection to the probe, the probe mount having a port forming the second outlet, and a conduit passing, in use, from the tank through the first coupling to the port; a probe head for attaching to the top end of the probe and comprising an inlet for attachment to a pressurised gas supply and an outlet forming the first outlet.

The invention will now be described, by way of example with reference to the accompanying drawings which is an illustration of a tank pressure monitoring unit according to the present invention.

The tank pressure monitoring unit illustrated in the drawing is adapted to fit into a single hole drilled into the top of a tank. The unit comprises a bubble tube or pressure probe 11 which passes through the hole and is lowered into the tank to a predetermined distance so that the depth of the operative part of the probe in the tank is known. The probe is fitted to the tank by means of a probe mount 12 through which the probe 11 passes concentrically. The probe mount 12 is an adaptor for fitting the probe to the hole in the tank whilst allowing the reading of pressure above liquid in the tank through the same hole. The probe mount comprises a first coupling in the form of a tubular portion 13 which is threaded on the outside at 14 to make a sealed connection with the tank.The inside of the portion 13 is spaced from the probe to provide a conduit between the area above liquid in the tank and a chamber 15 in the probe mount situated outside the tank so that the gas pressure in the chamber is equivalent to that at the top of the tank so that a pressure reading of gas in the chamber is substantially equal to that in the tank. The chamber 15 is disposed in a mount arm 16 which is branched from the body of the probe mount. An optional second arm 17 is blocked off by a plug 18 in this application, but is available for use if it is required to attach a supply of gas under pressure, or to vent excess pressure via a safety valve.

A second coupling is formed by a sealed connection made between the body of the probe mount and the probe by means of an olive 19 and a lock nut 21. Further details of the probe mount and the connection of the mount to the tank can be found in United Kingdom patent specification No 2 207 723A.

The top of the probe 11 is sealed off by a probe head fitting 22 being generally T-shaped with first and second arms. A first arm 23 is connected via a tubing nut 24 to an inlet line 25 for gas under pressure. A non-return valve 26 is fitted to the inlet line 25. The other arm 27 provides a pressure reading for the probe at the known depth in the liquid. This arm 27 is connected via a tubing nut 28 to a tube 29. The probe mount 12 and the fitting 22 are orientated with respect to one another so that the arm 27 is positioned over the arm 16. A differential transducer 30 is disposed between the arms 27 and 16 with the positive side connected via a female adaptor 38 to the tube 29 and the negative side connected directly to the top of the arm 16 in communication with the chamber 15.

An electrical signal representing the pressure difference at the transducer 30 is output from the unit through an electrical wire 31 from the transducer which is connected to a twin core screened cable 32 at a water proof cable joint 33. This signal may be carried by the cable 32 to a remote control room without any risk of contamination from toxic gasses in the tank.

The above unit can be modified by the provision of a second pressure probe (not shown) if it is required to determine both the pressure and density of the liquid in the tank. The second probe would usually be mounted concentrically with the first probe 11.

With the transducer mounted directly to the probe mount of the unit the only signal which leaves the environs of the tank is the electrical pressure difference signal which is entirely free of toxicity. No vapour can be transferred to the operations location.

There are various advantages that result from the use of this unit. These are listed as follows: 1) There is no need for a "wet leg" because if vapour from the liquid in the tank reaches the bottom of the transducer it drains back. Because of the pressurised air flow for the probe, under normal operating conditions no vapour would be able to reach the top of the transducer.

2) If there were a fault in the pressurised air supply to the arm 25, the short length of tubing that exists between the probe head and the transducer would mean that an accumulation of vapour in this length would not affect the pressure reading.

3) There is no need to empty the tank when a unit is being fitted.

4) There would be no corrosion of the transducer diaphragm under normal operating conditions as there is no liquid present at the transducer ports.

5) The transducer is easil replaceable should this prove necessary.

6) In dangerous environments it is possible to use an intrinsically safe transducer.

7) More accurate readings can be taken using an intelligent or "smart" transducer.

8)The unit can be installed,even in dangerous environments using breathing apparatus, using a 1" hole which is often pre-drilled in tanks.

Claims

1) A method of monitoring the liquid contents of a tank comprising delivering a first pressure value representing tank liquid pressure at a known depth to one side of a differential pressure transducer and delivering a second pressure value representing tank pressure above the liquid to the other side of the differential pressure transducer wherein the differential pressure transducer is located at the site of the tank and the method includes the step of delivering an electrical signal representing the difference between the first and second pressure values from the tank location to a remote operations location.

2. A method according to claim 1 substantially as herein described with reference to the accompanying drawings.

3. A tank pressure monitoring unit for fitting to the top of a tank for carrying out a method according to claim 1, the unit comprising a pressure sensing probe for delivering a first pressure at a first outlet representing the pressure at a known depth in the tank, a conduit for delivering a second pressure at a second outlet representing pressure above the liquid in the tank, wherein the unit includes a differential pressure transducer; the first and second outlets are fitted to respective sides of the differential pressure transducer; and the output from the unit, in use, is in the form of an electrical signal from the pressure transducer.

4. A unit according to claim 3 adapted so that the differential pressure transducer is arranged at a sufficient angle from the horizontal with the lower end attached to the second outlet so that condensation of vapour from the tank does not accumulate at the transducer terminal.

5. A unit according to claim 3 or 4 including means for delivering the first and second pressure values from the tank through the same hole in the tank.

6 A unit according to any of claims 3 to 5 the unit comprising a probe mount having a conduit there through to receive the probe and a first coupling adapted to make a sealed connection with the tank while allowing the probe to pass through the first coupling and through the hole in the tank, the mount including a second coupling for making a sealed connection to the probe, the probe mount having a port forming the second outlet, and a conduit passing, in use, from the tank through the first coupling to the port; a probe head for attaching to the top end of the probe and comprising an inlet for attachment to a pressurised gas supply and an outlet forming the first outlet.

7. A unit according to claim 6 wherein the transducer is physically located between the mount arm and the probe head.

8 A unit according to claim 6 or 7 in which the transducer is fitted to the mount arm.

9. A unit according to any of claims 6 to 8 wherein the said port is disposed in a mount arm, which mount arm also provides at least part of the said conduit from the tank to the port.

10. A unit according to any of claims 6 to 9 wherein the conduit includes a chamber in the mount arm adjacent the port.

11. A unit according to any of claims 6 to 10 wherein the probe mount includes a tubular portion with which the first and second couplings are made and through which the probe passes leaving a clearance with the tubular portion at the first coupling to form the said conduit through the first coupling.

12. A unit according to any of claims 6 to 11 wherein the probe mount has a second port through which pressure readings can be taken or through which gas can be supplied.

13. A unit according to claim 12 as appended to claim 11 wherein the second port is formed in a second mount arm connected to the tubular portion.

14 . A tank pressure monitoring unit substantially as herein described with reference to the accompanying drawings.