Gas Basics
We all know that pressure and flow are commonly used terms in domestic wet heating systems, but a lot of people don't understand the basic science behind pressures and flow rates. Here were going to specifically look at Gas, although most of the fundamental ideas remain the same for water.
What is pressure?
A gas of any type or mixture, is made up of molecules, which can be elements like Oxygen (O2) or compounds (different elements bonded together) such as Carbon Dioxide (CO2). In their gas state they are free moving molecules moving about bouncing off each other and surfaces at very high speeds, to give an example the molecules of gas in the air at normal room temperature will be traveling between 400 and 500 meters per second. The warmer these molecules get the faster they move, this higher energy state is what we feel against our skin as an increase in temperature. But in a fixed volume, say an air tight chamber or a static gas pipe, by adding additional energy (heat) to the system increases the speed of these molecules, with nowhere to go and the molecules hitting off each other and the inside surface of the chamber faster and faster exert more force against each other which is what we measure an increase in pressure.
Volume, Temperature and Pressure are all linked together and the effects can be seen every day in heating systems, refrigerators, heat pumps, all of which use these basic scientific principles to operate and transfer energy.
There are a few "Laws" in science that you might have heard of before that are relevant to this subject. Boyles Law: Which covers the relationship between pressure and volume. Charles' Law: the relationship between temperature and volume, and Gay Lussac's Law: The pressure temperature law.
To help visualise what's happening in each of these laws let's talk about a few examples, I'll state some rough figures to make it easier to picture what's going on without getting too bogged down in maths and calculations.
Example 1: Boyles Law
I take a typical large medical syringe that holds 100ml, draw the plunger all the way out allowing gas to be drawn into the syringe, the gas is at atmospheric pressure (1bar) I then cover the end of the syringe with my finger to create an air tight gap.
Boyles Law says if I press the plunger down on the syringe till there is only 50ml of gas in the syringe, then the pressure must now be 2bar, if I keep pressing the plunger down further to 20ml we will have 5bar of pressure in the syringe, and further more to 10ml I'll then be trying to hold 10bar in there.
It's important to note, that the amount of gas in the sealed syringe has remained constant, I have squeezed it into a smaller space reducing the volume it occupies, but there is no less gas inside it, the gas molecules now banging off each other and the walls of the syringe in a more compact space is what's increasing the "pressure" we would measure inside it.
Example 2: Charles' Law
If we had a sealed balloon that held 10 litres of gas at 20°c, and were to heat it up to 100°c, the gas would expand and now fill the balloon taking up a volume of 12.7litres. If we kept heating the gas further, all the way up to 1000°c the volume of the balloon would be 43 litres.
Example 3: Gay Lussacs' Law
We have a sealed copper gas pipe with a fixed volume. At 20°c the pressure in the pipe is 1bar, and we heat the pipe up so that the gas inside is now 40°c, the pressure in the pipe will have risen to 1.07bar, heating further to 1000°c would increase the gas pressure to 4.3bar. Similarly if you had the same gas pipe at 20°c, except this time there is 100bar of pressure in it, if you were to drop the pressure in the pipe to 50bar, then the temperature of the gas would also drop, to -126°c
While the above examples are each only taking into account 2 of the 3 aspects of pressure, temperature or volume at one time, in real life the 3 are always involved and the numbers become slightly more complicated.
In real life you put your finger over the end of the syringe and squeeze down to compress the air inside, the pressure will go up significantly and the temperature will also increase very slightly. If you heat the air in a balloon increasing its temperature, the volume of gas inside will increase as well as the pressure.
The big difference between gas and water when it comes to pressure, is that water cannot change its volume very significantly under pressure, even deep in the ocean where the pressure may be 1000bar (1000 times the atmospheric pressure we feel standing on the beach), the volume of a specific amount of water will have only changed by less than 2%.
Why does it matter?
It helps to have a basic understanding in pressure when working out problems on heating systems or sizing up new installations.
Equilibrium: A state of a system where all things are balanced. It's particularly helpful to remember that this balance is what is always trying to happen, for example high pressure will always try to travel to lower pressures and become an equal pressure through a system, heat will always try to travel to cooler areas till they become balanced at one temperature.
Anyone working on domestic gas pipework will be aware of a gas tightness test. A test where you add gas to pipework in a house, raise the pressure above atmospheric pressure and cut off the supply. Gas at higher pressure will always try to get out to mix with lower pressures and equalise. So the idea of the test is that any failure of the pipework or connections will appear as a pressure loss on a pressure gauge attached to the pipework, if the pipe is sound then the gas inside will stay at the pressure it was filled to initially.
Part of the test is what is referred to as the "Pressure Stabilization" period. Where the fill gas is left in the pipework for a minute before being topped up if necessary prior to starting the pressure test.
One of the reasons for this pause is directly linked to the laws above. If the gas entering the system through a mains gas pipe is cold and it fills the gas pipe in the property which may be significantly warmer than outside, then we will have a fixed volume of gas increasing in temperature as it warms inside the pipework. Increasing the temperature of a gas will increase the pressure, so if we didn't allow for the temperatures to stabilise, the pressure could rise during the test.
The danger comes where we have a small leak on the gas pipework. If the test should show a pressure loss from a leak, but the pressure is still rising from being heated, they could cancel each other out. You could miss a gas leak on the system because the pressure remained steady.
The opposite effect can commonly be seen when testing pipework very shortly after soldering a joint. The excess heat applied to the pipework during soldering takes time to dissipate. If you fill the system with gas for a pressure test, you often can see the pressure dropping during the test — not because you have a leak, but because the pipework and gas inside it is cooling down.
What are you testing?
When we work on Natural Gas installations, we have a number of gas pressures we need to check and understand their significance.
Standing pressure. Standing pressure is the pressure in a gas pipe when no gas is being consumed by any appliance. Pressure always wants to equalise so it will be the same wherever you check on an installation. So if the standing pressure at the meter outlet is 20mbar, then the standing pressure at the boiler gas valve will also be 20mbar.
Against a total vacuum, the pressure at the meter would in fact be closer to 1020mbar. This is the reason why when you have a faulty regulator or turn the pressure down at the regulator, you wouldn't use much less gas. While 10mbar may be half the amount of gas in the pipe compared to 20mbar, 1010mbar would still have 99% the amount of gas in it as a pipe with 1020mbar. To keep things simple (and for more technical reasons), we always measure gas and water pressure relative to atmospheric pressure.
Working pressure. Working pressure is the gas pressure in the system while an appliance is working, usually referenced when the appliance is working at its full potential load. This pressure will be different depending on where on the installation it is measured from. For example, you may find when a combi boiler is running on full rate, that you have 20mbar working pressure at the meter outlet, but on testing at the appliance gas valve you only have 17mbar. This is because the gas has to move from the higher pressure to the lower pressure area to equalise, but there is friction in the pipe that slows the movement of the gas down.
This becomes a particular problem when you have multiple appliances on the end of a long gas run. Let's say you have a boiler and a cooker teeing off at the end of the pipe. When you light up a couple of hob rings, the pressure at that end of the pipe drops and gas flows from the higher pressure at the meter to equalise. The hob rings use a small amount of gas, so the working pressure measured at the hob might only drop from 20mbar to 19.5mbar.
However, when you fire up a 30kW boiler, the pressure drops significantly at the end of the pipe run, requiring a lot more gas to flow along the pipe to keep pressure equalised. If the run is very restrictive, the gas cannot get down the pipe quickly enough to maintain pressure at the far end, and the working pressure may drop to only 5mbar. This is where it can become dangerous.
Modern boilers have flame sensing and would typically cut out when there isn't enough gas to burn. But many cookers and hobs still don't have FSD protection (Flame Supervision Device). If pressure drops so low that there isn't enough gas flow to keep a stable flame, the flame extinguishes at the hob. Then when the boiler stops firing, pressure rises again and unburnt gas can escape freely from the hob ring.
This is one of the reasons pipe sizing and correct testing of new gas installations are so important. The most common failure of new installs with regards to gas safety is low working pressures due to incorrectly sized pipes or overzealous use of flux — which unlike on heating pipes, is never flushed from a gas install and becomes a permanent resistance in the pipe.
