GAS TRAIN

What does Gas Train comprise of?

It mainly comprises of a:

1. An isolation ball valve, 
2. Filter, 
3. Pressure-regulating aggregate consisting of safety cutoff valve, Pressure regulator, safety 
4. relief valve
5. Inlet & outlet pressure gauges for the pressure regulator 
6. Gas Flowmeter with bypass arrangement (optional)

Where is a Gas Pressure Train required?

The Gas burners are fitted on a boiler suitable for Gas specified which can be Natural Gas, LPG, Propane, Butane, etc. These burners are connected with two gas solenoid valves into which the gas line is connected for supplying gas to the burner. The pressure required for gas firing at the burner tip is varying between 15 mbar – 100 mbar for different gases.

The gas available at the site is at a different pressure than the required pressure for firing.  Therefore it is required to regulate the gas available at the site to the required pressure for firing. The gas train is connected between the gas line and the gas solenoid valves of the burner.

When to choose a Low-pressure Gas train & a High-Pressure Gas train?

Operating pressures required by the burner solenoid valves is up to 0.5 bar are called low-pressure supply, whereas operating pressure > 0.5 bar is classified as medium to High-pressure supply.

Normally when the consumer is getting a gas supply at a pressure between 1– 4 bar. The High-pressure gas trains are available for an inlet pressure range of 1- 4 bar. 

 What will be the case if inlet pressure exceeds 4 bar?

In such cases, High-pressure gas trains are connected in series to reduce the pressure,. First from rated inlet pressure (more than 4 bar) to 4 bar,
Second from 4 bar to required pressure at the burner gas solenoid valves. 

What does Low-Pressure Gas Train consist of?

The low-pressure gas train consists of only an isolation valve, a gas pressure regulator, and pressure gauges in the inlet and outlet of the regulator. Also, the connections are all screwed connections as the gas pressure is low. The size of the regulator is normally the same as the size of the gas solenoid valves of the burner. 
The cost of a low-pressure gas train is low and it has to be offered where the inlet gas pressure is 300 mbar or less.

What does High-Pressure Gas Train consist of?

The high-pressure train consists of all the components mentioned in the low-pressure gas train. But all the components are flanged instead of screwed. Also, it consists of a pressure regulating aggregate instead of a pressure regulator as in a low-pressure supply. This aggregate consists of a Safety shut-off valve and Safety relief valve assemblies. These are required to protect the regulator and burner solenoid valves from excess gas pressure. 

The High-pressure gas train also consists of a flexible bellow flange i.e a compensator. This is fitted between the Pressure-regulating aggregate and the Gas Solenoid valves at the burner.  This absorbs the shocks due to the vibrations created in otherwise a fixed gas line due to switching operations of the burner gas valves as per the varying loads on the burner. 

What should be the features of the Gas pressure regulators in the high-pressure gas train?

The gas pressure regulators are subjected to very high dynamic demands owing to the speed of switching operations of the burner’s gas valves and the small volumes of gas between the gas regulator and the safety shut-off valves of the burner.
 
The gas pressure regulators need to react quickly to the opening/closing times of the safety valves as well as burner load changes during operation. It must also react to emergency shutoff of the burner from full load operation. In this case, the safety shut-off valve is usually activated. The gas pressure regulator ensures virtually constant gas pressure to the burner across all load points irrespective of the gas inlet pressure within the range of the unit. If outlet pressure exceeds the set value or is under zero flow conditions, the gas regulator will close its throughput orifice. 

What is the purpose of the safety shutoff valve?

Safety shut-off valves serve as a primary safeguard against gas pressure and throughput. During the Normal burner operation, this is open. It shuts off the gas supply if its set pressure has been reached.  Resetting has to be done manually.  Via an impulse line, the Safety shutoff valve senses the outlet pressure from the pressure regulator section and if this pressure exceeds the set value of the SAV, the SAV shuts off the flow of gas entering the gas train. 

What is the purpose of the safety relief Valve?

If the internal gas leakage is detected i.e the gas pressure regulator does not close its seat, the safety relief valve vents the excess pressure to the atmosphere. The excess pressure increase is also possible if the gas pressure regulator supplies an excess outlet pressure due to faulty operation or if a Safety shut-off valve does not close its seat and leakage via the seat occurs. A vent line to the atmosphere should be provided to ensure that possible internal gas leakage can be vented safely. 

What are the function of the Ball Valve and Gas filter?

Ball valve provides an isolation facility to the system from the main gas line for any maintenance etc. A gas filter ensures clean gas without any dirt to the other components of the gas train following it. Gas filters and ball valves are selected not to exceed a pressure drop of approx 50mbar.

What is the function of inlet & outlet pressure gauges with shutoff valves on the Pressure regulator?

The pressure gauges indicate the inlet gas pressure and regulated gas pressure after the pressure regulator on the gas train and give indications for any faulty operation of the regulator.

What is the function of a compensator?

The gas train is connected on one end to the gas supply and on the other end to the gas solenoid valves of the burner.  The complete connection is flanged and fixed. There are vibrations in the line due to the switching of the gas solenoid valve of the burner and also on the emergency stop of the boiler. This vibration is absorbed by the flexible bellows connected between the gas regulator and burner solenoid valves.

Why are reducing flanges required sometimes in the high-pressure gas train?

In many cases, the sizes of the inlet and outlet of the pressure regulators are smaller than the gas solenoid valves, particularly with high gas pressure. In such cases, reducing flanges are required in the high-pressure gas train.

What should be the distance between the gas solenoid valves and the gas pressure regulator?

It is important that gas solenoid valves are very near to the burner (less than 1 mt) but the distance between the pressure regulator and gas solenoid valves can be small or several meters.  With a large distance, the gas flow can be stabilized and buffer volume is obtained. 

How is the cost of the high-pressure gas train over the low-pressure gas train?

For the same burner throughput, the cost of a high-pressure gas train is 6-8 times higher than the low-pressure gas train.

What to do when there are two or more burners at the same site?

In such cases, we have to design a high-pressure gas train for rated inlet gas pressure available and for total gas quantity to be consumed by both the burners. Further after the common high-pressure gas train, it is to be connected to the individual burner through an individual low-pressure gas train (filters can be avoided in the low gas pressure train in such cases). This will be cheaper than separate high-pressure trains to each burner.