Currently HRSG (NG+PG fired, 30 tons/hr at 105 kg/cm2 & 510 oC) is being operated with following steam pressure controls:

1. Cutting of one gas firing at 107 kg/cm2 and shifting of control mode from flow to pressure.

2. At 110 kg/cm2, HRSG trips

3. At 112 kg/cm2, PSV lifts

We are considering to configure automatic opening of Steam Vent Valve through DCS to control steam pressure rise, if any.

Any suggestion, expert opinion? I shall be grateful.

 

You did not indicate whether the pressure rise is sudden, eg due to the trip of a steam turbine fed from this HRSG, or the pressure rise is relatively slow, eg due to the unloading of a steam turbine, and the boiler not managing to follow the reduction in steam demand.

In the first case, i.e. sudden pressure rise, there is not much you can do, as any control system together with the boiler's thermal inertia will not be fast enough to control the situation, and the opening of the PSV is unavoidable.

On the other hand for a relatively slow pressure rise, you may try to use the Steam Vent Valve, but you need to be careful.

1. Sudden opening of this valve will result in a collapse of the boiler pressure, resulting in the swelling of the water inside the boiler drum, with the possible consequent trip on high drum level.

2. Again opening slowly to circumvent condition (1) above may not be fast enough to control your boiler pressure.

3. Is the Vent Valve sized to handle up to the maximum steam flow from the boiler, or is limited?

4. How often do you get such pressure rises requiring venting? Keep in mind that every time you vent you are wasting make up water. Is there enough reserve to cater for these eventualities, and can your make up system cope with these steam blow offs?

5. Is your vent valve located after the boiler? When this vent opens will your superheater still have steam flow through it to prevent it from overheating, especially if your afterburner remains in operation?

6. Why are you ending up with these pressure rises? Can you instead control the process using the steam from the boiler from creating such pressure rises?

Some food for thought.

 

To add to jojo's points, another thing to consider is the noise that would be generated by opening the steam vent at these pressures, and what Environmental Permit parameters this may exceed.

One option maybe to use, if installed on your plant, a Bypass valve to bypass the steam from the HRSG to Condenser. At the CCGT where I work, the HP and IP Bypass valves have a function in their control logic to try and maintain a maximum pressure. For example, normal operating pressure maybe 100 bar, SRV set at 110 bar. The HP Bypass valve setpoint in normal operating mode will slide up to a maximum of 105 bar (to keep it closed in normal operation and prevent necessary dumping of steam), but will start to open if the pressure rises above this value, to try and prevent the pressure from increasing and ultimately the SRVs from lifting. Which of course are your last line of defense.

However, I don't know how easy this would be to implement with your control philosophy and your plant configuration...the HRSG at the plant I work at has no supplementary firing, heat is purely from GT Exhaust gas. Is your plant a CHP plant out of curiosity?