why steam turbine tripping provided for low inlet steam pressure

Comments You like this 17 comments

Joseph FJoseph F Byrd, JrHome health and personal care aide at Unpaid

usually a turbine trip is to protect it from an adverse operating condition. I have never seen a turbine with a low inlet steam pressure trip. 

however the units did have a control algoritham than would reduce steam flow in an effort to maintan pressure. (initial pressure limiter - IPL). this purpose was to reduce the chance of boiler water carry over if drum level control is lost. This circuitry would to limit CV opening in response to a frequency droop, which woulld in effect intiate a water carry over event 

Is it possible your turbine governor control does not have IPL so the easy solution was to just add a trip? Alfred Hertaeg likes this

RhodriRhodri EdwardsSenior Engineer at SKM

I suspect it's not actual low pressure that's tripping the unit but low steam mass flow. If your boiler is producing 100% of the steam at say 75% of the normal inlet pressure the steam volume will be much greater and the inlet will choke. In pactice this will not happen, your inlet pressure will rise until the inlet is not choked. However if your boiler is producing 50% of your steam mass flow at 75% then the turbine will run and probably not trip. What it sounds like to me is that your boiler feed pump is maintaining drum level but for some reason your boiler is not producing the mass flow of steam. If you don't have sufficient steam flow through your bladepath you will get windage which will cook the blades. Hence the trip function.

DeepakDeepak NayakSenior Manager - International Sales & Marketing at Triveni Turbine Ltd.

When inlet steam pressure drops the turbine governor gives command to the actuator of throttle valves to open up more (including the overload valve) to maintain the power output from the turbine constant. This is possible only up the extent of maximum area allowable with all the inlet throttle valves fully open. After this limit more inlet steam flow into the turbine is not feasible due to which reason the turbine frequency starts dropping as power load on generator exceeds the generation. Then the turbine actually trips on low frequency. However when the inlet steam pressure drops it is also often accompanied by loss of superheat and water droplets carry over which is dangerous for steam turbine as these droplets impinge with the blades causing pitting and also the droplets cause increased thrust load leading to failure of thrust bearings. Due to this reason the turbine manufacturers recommends low inlet steam pressure alarm followed by trip. Anil S., Svetoslav Katerinski and 1 other like this

AmitabhAmitabh SrivastavaAdviser at Jindal Power Limited

Some of the Steam Turbines are provided with "Initial Steam Pressure Unloading Gear (ISPUG)". The purpose is to start unloading of the Steam Turbine if the Main Steam Pressure drops to 90% value - at this point unloading starts by command to control valves to start closing. Thus reduction in steam consumption in steam turbine helps boiler operator to restore the main steam pressure in the boiler. If Main Steam pressure continuously goes on dropping then ISPUG unloads the set completely at about 70% value of main steam pressure. Normally no tripping is provided as it is not required. However, if it is not possible to maintain main steam pressure even upto the level of complete unloading of steam turbine then it is advisable to trip the set as there may be serious problem in the boiler. Er. Devinder Singh, Joseph F Byrd, Jr like this

RobinRobin DynesPrinciple Product Support Engineer at Allen Steam Turbines ( Weir Power&Industrial)

Allen turbines have had this protection for many years. We have both a Low inlet pressure trip and a Low inlet pressure over-ride in the governor that closes the inlet valves to try and maintain a minimum inlet pressure and protect the boiler. The valves close in to a minimum setting and then if the pressure continues to drop the set trips. Chris Comi, Joseph F Byrd, Jr like this

N SN S TomarADDITIONAL GENERAL MANAGER at BHEL, PSTS , NOIDA

Dear Kiran Good learning by the comments of well experienced engineers. Thanks to all of them. I had been involved in operation of a machine which has Low Steam pressure un loader, but we kept it isolated. We were more serious about inlet Steam temperature Low and some time machine tripped due to sudden temp drop. Er. Devinder Singh likes this

AlfredAlfred HertaegSteam Turbine Engineer

My understanding of low steam pressure unloaders and trips is to protect against water carry over flowing a major boiler problem. Haitham Farghli, Rodney Buttner like this

RobinRobin DynesPrinciple Product Support Engineer at Allen Steam Turbines ( Weir Power&Industrial)

Alfred, your comment is correct but we also install a Rate of Change of Temperature monitor in the primary feed from the boiler to give a turbine trip signal if a "slug" of water has been released from the boiler. This device has saved the turbine from damage on several sites. Haitham Farghli likes this

ManoranjanManoranjan MohapatraMechanical engineer

Steam inlet pressure of the turbine also effects the turbine performance. All the turbines are designed for a specified steam inlet pressure. For obtaining the design efficiency, steam inlet pressure shall be maintained at design level. Lowering the steam inlet pressure will hampers the turbine efficiency and steam consumption in the turbine will increase. in turbo driven Pumps if the inlet steam pressure below the rated it will increases the steam demand and control valve in 100% opening

condition .It can't achieve the rated speed as required .hence the drive will tripped due to low steam pressure. Haitham Farghli likes this

RodneyRodney ButtnerSenior Mechanical Engineer, Seeking immediate employment - Mechanical , Project , Reliability , Maintenance

I agree that the steam inlet pressure to a turbine does affect its performance. However, Alf's comment is correct as water carry over is an issue following a major boiler problem. In the right circumstances, it is appropriate to operate the steam turbines at lower than design pressure in order to stop the control valves from operating in an undesirable low range. eg load turn down overnight.

Sanjay KumarSanjay Kumar SahTechnical Leader at GE Aviation / Oil and Gas

Pressure is directly proportional to mass flow rate. As pressure come down, mass flow rate will also come down resulting into less power production. It will impact RPM of the turbine also which will affect frequency of power generation. Change in frequency will impact badly to Grid power. Hence for precautionary measure, turbine tripping provided for low inlet steam pressure

N SN S TomarADDITIONAL GENERAL MANAGER at BHEL, PSTS , NOIDA

In my opinion, reduction of inlet steam pressure to steam turbine results in reduction of power generation by this turbine and RPM of this turbine shall not change individually as far as it is connected in grid .If at all, frequency of grid changes , the other turbines connected in grid shall pick up or unload depending upon the grid frequency, if their Governors are kept on FGMO mode. Chris Comi likes this

RamkumarRamkumar MGAssistant manager(O&M) , 2x80 MW powerplant-Chennai,India

The best answer given by Mr.Deepak Nayak. 1.The mass flow rate increases 2.Due low pr., turbine tends to draw more steam from boiler which leads to water carry over and causes further drop in boiler pressure.water particles causes blade damage. 3.This results in RPM drop causes under frequency trip.

SHAREENASHAREENA ABDULLAHOIL & GAS Facilities and Equipment Maintenance Professional

Low inlet steam pressure indeed maybe not recognised by the governor, only speed and driven load. Low pressure steam will ended up halfway at the intermediate stage of blades/bukcets or nozzles whereby vapour turns to condensate and erosion and blade excitation can initiate crack or high vibration to rotor and potential rubs..

AlfredAlfred HertaegSteam Turbine Engineer

For a Utility type turbine that is only used to generate electric power low steam pressure is not a problem these turbines can operate in sliding pressure mode with governor valves near wide open to maximise efficiency. You need to protect the turbine against a rapid drop in steam pressure or steam pressure below set point which indicates a boiler problem that may result in water ingress. 

Mechanical drive turbines that are connected to pumps or compressors for example may need a low pressure trip when there is not enough steam available to keep the turbine running near normal operating speed. 

Turbines also need to be protected against low steam temperature or water detected in steam piping. Normally thermocouples are too slow to prevent a water slug entering the turbine once it ha formed. Velocity in steam piping is approximately 50 m/s and thermocouple time constants are in the order of 20 seconds. A water slug can travel a long way before a thermocouple responds. The low steam temperature protection that I am familiar with monitors conditions where water slugs can form. Robin's rate of temperature change detection overcomes thermocouple time constant problem but I have no experience with this type of protection. The trip settings would need to be set so that it closes the turbine stop valves before the water reaches the turbine. I am more familiar with direct water detection using resistivity probes or similar. Rodney Buttner likes this

SHAREENASHAREENA ABDULLAHOIL & GAS Facilities and Equipment Maintenance Professional

Protect reverse flow mostly for back pressure steam turbine. Blades reversed loading can affects number of cycle to fatigue.

kirankiran lenkaOperation Power Plant at OSIL

thank u for all comments