Dear Kalimuthu

In steam power plants it is not unusual to decrease the steam flow of one or more extractions in different operational conditions. However, if you want to do that not based on operational instructions, it will have some side effects.

For example, in feedwater heater repowering where you block some extractions from steam turbine, the turbine will face some side effects. The main one is increasing the losses of steam path in it. Suppose flow through a stage of turbine is 'M' and before the second stage we extract from flow a value (E). So the 2nd stage has been designed for flow of 'M-E'. If you block the extraction flow then the flow through second stage (and other stages in downstream) increases by 'E' and will be 'M'. The side effect is changing the velocities triangle. In brief, absolute velocity of nozzle's exit at downstream stages will increases that is equivalent with higher flow angle in inlet of moving blades. Therefore, these stages will face with higher incidence loss.

The overall effect is lower efficiency of steam turbine if we do not apply any design modifications. Although the efficiency drop should be calculated for a specific case, in most cases it is not so large and we accept the deviations from designed velocity triangle.

I have worked a bit on this previously but my outcomes are not in English. Perhaps I could help you better if I know your problem exactly.

Thank you very much for your detailed explanation.

The answer to your question is yes and no based on the situation of the shutdown.

If there was not provision (with multiple valves for flow diversions) on the initial design of the specific turbine, it will be difficult if not impossible the short completely short off the steam extraction for FWHs without affecting the performance of each individual component as well as SAFETY of the installation. More importantly perhaps the condenser and the feed-pumps were not initially sized to safely deal with such scenario.

As long as the steam mass flow rate extracted to feed the FWHs is reduced and remains within the minimum value prescribed by the manufacturer specifications, the installation should operate normally.

Very briefly (i) please advise what you intend to achieve by shutting off the steam flow. I would assume that the OEM or initial plant designers knew their job.

(ii) Do you know the design flows for each section of the steam turbine and boiler? The feedwater heater has a function so reducing its steam supply will have an effect.

(iii) Do you know the pressures and temperatures of the various streams?

Has the plant been modified already and changed from its initial design?

(iv) Is this flow minor when compared with the main steam flow? is it sufficiently low to have minimal impact on the main steam flow?

(v) Are there already any off-design operating conditions which must be met and are these critical and/or will these occur frequently? Please remember that heat transfer coefficients of condensing steam are far higher than those for superheated steam or single phase water, all other things being equal. Also letting down steam from high pressure may lower its temperature but will usually increase its degree of superheat.

I don't like answering questions with questions but help here will be beneficial. Theoretically it can be done but only after a system appraisal.

Regards Stephen.

The complete closure of the high-pressure heaters is a measure of temporary increase of the power generated by the machine. During this regime it happens what very good described Hamid Abroshan . That means the stages operates at lower efficiencies than rated. Another unwanted effect is that the boiler water supply is pumped in the steam generator economizer with a lower temperature (160-170 degrees Celsius) than the rated one (imposed by the saturation temperature in the last HP heater). This conduct to a higher fuel consumtion.

The danger is to achieve the critical regime in the minimal sections of the nozzles (usually convergent), that means the steam flow trough it cannot increase anymore and the turbine is uncotrolable.

The partial closure of the high-pressure heaters is impossible while there are not control valves or rotary diaphragms on the uncontroled extractions.

Arash Toudeshki - Thank you.

Zacharie Tamainot-Telto - Thank you for your insight.

Stephen Lloyd - Thank you for putting up more think points to understand better. I will answer you in the subsequent comment.

Gabriel Negreanu - Thank you for your interesting explanation.

I think that the above comments should answer your questions on the whole, but I will add a couple of observations. Forcing extra steam through a turbine casing is not usually a good idea unless the steam turbine is designed to accept it. Pressure drop may increase and the steam conditions at the inlet should be checked. Also please ensure that there is always adequate steam flow as with multi-stage machines with low flow through one stage, overheating in that stage can occur through churn. Stephen.

In actual plant, sometimes, top feed water heater is being withdrawn from service at full load due to low MS temperature or HPH leakages conditions. It will improve the plant performance at low steam temperature condition due to improve in steam quality at HPT inlet and decrease in flugas exit temperature. However, special attention to be given at very low fluegas exit temperature for avoiding acid corrosion in the duct. But short time problem on impingement loss in low pressure turbine could be avoided by reducing steam flow through turbine for maintaining same demand as load increased above the set value due to HPH withdrawn.

But, partial withdrawl of heater normally not recommended from maintenance point of view.

As a operation and efficiency point of view, it may be concluded that the effectiveness of top HPH in service is more when MS temperature cross the design limit and it will help to reduce the SH spray water requirement i.e. irreversibly during mixing at de-superheater section.

Goutam Khankari Thank you sir for the interesting insight.

Steam used for feed water heating is taken from bleed of Turbine.Turbine power out put depends on the quantity of steam extracted from various stages ofTurbine.So,if steam flow from bleed reduced or completly stopped, then total inlet steam flow will reduce & hence power out put.

However, if Turbine is running on partial load, there you can reduce or completely stop the bleed steam.As this quantity of steam can be extracted from next stages or it will goes to condenser.

Conclusion: Bleed steam is uncontrolled steam, it dpends on the Turbine load.And we can reduce or stop the bleed steam based on steam Turbine load.

Also refer

https://www.powerplantandcalculations.com/2020/07/30-things-you-must-know-on-steam-turbine-auxiliaries.html

https://www.powerplantandcalculations.com/2020/08/how-do-you-calculate-power-generation-in-steam-Turbines.html

Mahesh Wali Thank you very much for your insight sir.

Yes. It is possible and it happens too. If any feedwater heater drip level goes high, the extraction valve automatically closes to save the turbine from water ingress. For that moment, the turbine will generate more power but slowly, the condenser heat load will increase and vacuum pressure will deteriorate resulting in efficiency loss.

Sourav Bagchi Thank you very much for the insight sir. I got the point.