Relation Between Pressure and Flow

Your regulator and/or line is too small....

Flow is proportional to the square root of pressure difference.

Regulators are sized for both pressure and flow rate.

A regulator can regulate to a given outlet pressure, but only have a limited capacity to supply a flow volume above its maximum design flow rate.

Whoever sized your regulator only paid attention to the pressure range, not to the flow rate capacity rating.

There is no better description of the pressure/flow relationship, in my opinion, than the Bernoulli equation:

http://www.princeton.edu/~asmits/Bicycle_web/Bernoulli.html

It explains or helps to explain more of the everyday pressure/flow problems than any other single equation. Get to know and understand it and it will answer pressure/flow questions for the rest of your life.

Here's a thought experiment; fill a glass with water from a syringe with a small bore needle attached. ( high pressure + low flow )

now try filling a glass of water from another glass filled with water. ( low pressure + high flow )

Try to work out what causes the difference.

Hint; don't get bogged down on ratio of flow to pressure, look at the remaining characteristic.

To explain your problem I am going to use filling a bucket of water as an analogy. Imagine that you have a tap at the bottom of a hill (your burners) and a lake at the top of the hill (external gas supply) with a connecting pipe. When you turn on the tap the pressure is so high so the water shoots past the bucket and does not fill it. You can solve this problem by adding another tank at the base of the hill with a ball cock. Now the water flows down the hill into the tank and when it is full the flow stops. If you open an outlet valve on the tank to a pipe connected to your tap the head is lower so the pressure is lower. The water now flows into your bucket. (You have added a pressure regulator.) But your bucket is not filling fast enough so you put in a bigger outlet valve, pipe and tap between the tank and the bucket (equivalent to fitting a regulator with a larger flow rate). Your bucket now fills for 30 seconds at the required flow rate then slows to a dribble. (the situation you are observing) What has happened is that while you were filling from the volume in the tank you have adequate flow, but when the tank empties the tank is only delivering the amount of water that is flowing down the hill. It is passing through the tank so is at lower pressure, but the flow is determined by the size of the pipe between the lake and the tank. (This is the rate of the external gas supply available) Because it was flowing at the correct rate for 30 seconds you know that your regulator is sized OK and does not need increasing. What you need to do is talk to your gas supplier to see if they can upgrade the supply available. Alternatively if your gas use is intermittent, you could increase the size of the tank so that it does not empty over the time you are filling the bucket. This is equivalent to putting a gas accumulator prior to your regulator. There are lots of rules associated with storing gas so this option should only be contemplated if the cost of upgrading the gas supply is too high.