Gas Drainage Flowmeter

This report describes the development of a flow meter for gas drainage operations. It is designed to be inexpensive and to be able to be monitored automatically utilising intrinsically safe electronics.

Description of the Unit

The flowmeter is robust and operates over a wide range of flows. It can accommodate water flow in the gas stream, survive being flooded and cope with some particulate matter.

The flowmeter has a surprisingly large flow range. It is expected that the production 2" (50mm) version will be able to measure flows from 2 to 150 litres per second (measured on the basis of air at STP conditions) at an accuracy of 2 to 3 percent of the flow measurement throughout the flow range. This is far better than existing flow meters in which the accuracy is measured as a percentage of full scale.

The flow meter also monitors gas pressure and hence provides information on the operating pressures of the gas drainage range. This is an important operating and safety benefit. The flowmeter is now being advanced into the production phase.

The Problem

Since the inception of gas drainage in underground coal mines there has been a need to monitor the amount of gas produced from the boreholes. There are a number of reasons for this monitoring, namely:

• To establish that the gas drainage borehole initially works.

• To find out whether the gas drainage borehole blocks with time.
• To establish the amount of gas left in the block of coal by the material balance calculation.

To date monitoring has been substantially achieved by hand, using orifice plate flow meters that are manually placed in the line from the standpipe to the gas drainage range. Such monitoring is too expensive to be undertaken at a desirable frequency.

An automatically monitored flowmeter for gas drainage applications is therefore highly desirable and was the subject of an ACARP application by Sigra Pty Ltd in 1995. The application was successful and the project commenced on 1 April 1996.

Objectives

The objective of the project as stated in the project application was to develop a borehole flowmeter that is suitable for gas drainage applications in underground coal mines. As such it must be robust and cheap enough to be left in the goaf.

To be able to meet this objective the flowmeter produced had to be able to monitor a wide range of flows and therefore have a large turndown ratio (ratio of high flow : low flow). In addition it had to withstand salty water and particulate matter coming out of the gas stream. This objective has not changed throughout the project though a number of additional desirable objectives have been added. Of particular significance was the need to monitor gas drainage pipeline pressures as an indication of the state of the range.

Though not stated as an objective in the original funding submission, it was obviously very desirable to enable the flowmeter to be monitored automatically and the information obtained to be telemetered to surface. This substantially narrowed down the types of device that could be used.

Present Developments

The flow meter is presently being tested in what is hoped will be nearly its final form. This device is 51.7mm diameter and will have a softer spring than the initial trial version. As a result it is expected to have a flow capacity of about 150 litres/second (air at STP) for a 5 kPa pressure drop. The current tests will finalise the calibration curves for various Reynolds numbers and hence gas types and gas densities.

The flowmeters will be available as a unit incorporating a shut off valve to close down the borehole in the event of gas drainage pipeline damage and a proprietary water and particulate matter remover to protect the meter from foreign matter and enhance its accuracy.

Holeville Pty Ltd has obtained intrinsically safe approvals for its monitoring nodes. They are expecting approvals for the master nodes at the time of writing. The flowmeter is the subject of a broad international patent application.

Developmental needs

To be able to handle the vast amount of information derived from the flowmeters a real time simulation package is needed. This will relate the gas drained into roadways and into boreholes with gas pressure/gas content left in place. The basis of such a simulation package is:

gas initially in place - gas drained = gas left

though the equations used can be expected to be couched in terms of reservoir engineering models. Such models will significantly enhance accuracy by being driven by flow rather than pressure boundary conditions.  A system such as this will significantly reduce the burden of gas drainage monitoring.

Conclusions

A simple workable flowmeter has been developed for gas drainage applications. This meter has a wide range of flows that can be monitored. Accurate measurement will be able to be achieved from about 2 litres/second through to 150 litres per second (air at STP conditions) associated with a pressure drop of less than 5 kPa (500mm water gauge). An accuracy of 2 to 3% of flow measured throughout the range is possible. The meter will in addition, measure gas pressure and thus will be able to monitor the state of the gas drainage lines.

The meter will be available at about its budget cost of $500 each though electronics will cost more, however if care is taken they will be reusable. The electronics will enable multiple flowmeters to be connected together in a daisy chained line or other configuration which may include spur lines. This will be connected to a master node that can transmit to surface via its own dedicated line or communicate with a PLC with access to the data highway.