One of the worst experiences any plant can suffer is the challenge of starting up fired equipment that has been offline for a period of time. Figure 1 is a step diagram illustrates a typical light off cycle.
Often ignition fails the first several times because the fuel line has been filled with inerts to isolate it. If the timing on the purge is short this isn’t too big of a deal.
But if the firebox is larger the purge time could be 10 or 15 minutes. In such a case 5 or 6 ignition failures represents a couple hours of delay in startup time not to mention the time spent by the operators. To reduce or eliminate this delay the fuel gas header needs to be purged with fuel gas, but simply purging flammable gas to atmosphere or even the firebox is not a very safe practice. There is an automation solution to this dilemma.
Figure 2 represents a typical fuel gas header.
Here we see a emergency block valve (EBV), supplying fuel gas to a pair of manual burner valves and an automatic burner with an igniter. Typically the EBV is actually a double block and bleed but a single valve will serve for this example. Depending on the length and size of the header and the time allowed attempting ignition it may be several tries before the gas in the header becomes rich enough in fuel to ignite. Each failure should initiate a re-purge of the firebox.
To reduce this dilemma a purge valve (again, a double block and bleed) can be installed around the EBV with a restricting orifice or a regulator to create conditions that can be used to purge the inert gases out of the header.
The idea is to pressurize the header five or six times and then vent the volume of the header into the firebox to purge the header of inerts. By limiting the time the bypass is open and using a restricting orifice it is possible to limit the amount of fuel potentially leaked into the firebox. If the header fails to reach a minimum pressure within the first couple of seconds the purge is failed, and it is assumed there is a leak or a valve has been left open. If the header fails to reach the target pressure within the time it should take to press up through the orifice then the purge fails, and a leak check needs to be done.
It is not unusual for significant time to be lost trying to light fired equipment simply because the fuel header has inert gas instead of fuel. This technique can be easily and cheaply added to most equipment to mitigate this problem.
This post was written by Rocky Chambers. Rocky is a senior control systems analyst at Maverick Technologies, a leading automation solutions provider offering industrial automation, strategic manufacturing, and enterprise integration services for the process industries. Maverick delivers expertise and consulting in a wide variety of area including industrial automation controls, distributed control systems, manufacturing execution systems, operational strategy, business process optimization and more.
Maverick Technologies is a CSIA member as of 7/20/2015