We have two 80MW Steam Turbines of Alstom make. We want to perform a modification that if one ESV closes during normal operation, our Steam turbine keep running with one ESV.

We actually have been facing problems on one of the steam units that it has tripped several times because one of its ESV solenoid's coil burn due to which ESV closes and cosequently turbine trips. When coil of solenid burns and because of which one ESV closes then indication "Stem turbine set to reset" appears and m/c trips with in 5 seconds.

We now want to make a modification to increase T/B trip time up to 30 seconds so that whenever above said indication appears operator on duty has enough time to open T/B bypass valve manually and hence tripping may be avoided.
Now what we are afraid of about this modification is that 30 seconds delay time in tripping without bypass open may cause over pressue in system which may lead to some extensive damage.

Please advise
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A number of things need to be checked, before you try to operate the steam turbine with one ESV closed. These are:

1. Is the steam turbine designed to run with one ESV closed. Do the ESVs supply to steam to the turbine to a common entry arc, or does each ESV supply steam to a part of the arc of entry of the steam turbine? If the steam turbine is of the full arc admission (i.e. all ESVs supply steam to the full face of the first turbine stage) then, most probably you may operate with one ESV shut. Turbine designer will have to confirm this.

2. At the moment that the ESV is closed, the steam inlet to the steam turbine is reduced to a half of what was originally coming in (assuming the steam turbine has 2 ESVs). This will cause a sudden steam pressure rise on the boiler feeding the steam turbine. For sure that your boiler safety valves are opening up until the boiler firing rate is reduced to cater for this sudden reduction in steam demand, unless your steam turbine is equipped with an automatic bypass system, in which case this is definitely opening, dumping steam to condensor. Such pressure exertions will not do good to the boiler and other high pressure steam pipework. The steam bypass system is also having its toll on its lifetime, given the valve and pipework after is forced from practically ambient temperature to steam conditions repeatedly.

3. Has the cause of the repeated burning of the solenoid really been investigated? Is the coil, apart from being continuously energised, exposed to other sources of heat, like for example, a steam leak impinging on the coil? Is the solenoid placed close to a source of heat (a badly lagged pipe, or even the ESV neck (which is not always insulated)?

4. Did you try to reduce the holddown voltage on the coil, to reduce the dissipation in it, and thus extend its lifetime?

I would try to get the solenoid hold on reliably rather than work around the fault by getting the turbine and boiler handle the transients. The costs of repair simply do not tally.

 

This is another "Let's fix a hardware problem with a software solution!" probably devised by some plant manager or, worse, an operator.

Rather than solve the solenoid burning problem (which is probably caused by the Sourcing Department finding a "comparable" but "less expensive" (you get what you pay for!) solution), they want to try to wreck the turbine and/or the boiler with a software fix to the turbine control system.

Fix the cause of the failing solenoids. As jojo says, one problem may be that the "economizing" output of the control system which is powering the solenoid may be damaged, or the solenoid may require or be capable of using reduced holding current and the control system isn't configured (programmed and/or wired) to do so.

Oh, and leaking packing or damaged insulation shouldn't be a problem, right? (Then why even lag pipes or have packing leak-offs?)

You'd be best advised to ask this question of the steam turbine manufacturer for the best response; but I'll bet they say the same thing or similar: "The solenoid shouldn't be failing," or, "Haven't we told you about our improved solenoid for that turbine?"

If only one of the turbine is experiencing a burning solenoid problem, that's no reason to apply a software fix to both turbines.

From a recent 'Fortune' on control.com: "Everything is possible if you don't know what you're talking about." Many a good technician has been led down a bad path by unknowing management--and because it's technically possible to do such things, they consider it a challenge to do so since they rarely get a chance to make changes to the turbine control system.

Think twice about this; ask the steam turbine manufacturer and then ask the boiler manufacturer and then review the piping systems and steam bypass systems (again, as jojo recommended)--and then, *don't do it*! Fix the real problem, no matter how hard it might be. The easy path isn't always the best, or most correct, path.