## Steam Turbine performance with variation in inlet steam pressure

## Steam Turbine performance with variation in inlet steam pressure

(OP)

Dear All,

I am estimating a future problem while still in erection phase for 60 MW steam turbine. I may need your help to estimate a solution.

Due to some reason my process heater is not able to generate steam at the desired pressure (other parameters are remain same). Therefore I want to know the effect of

Steam Turbine is Superheated, Condensing type with Reheater (HP-LP combination)

1.

Manufacturer : GE

HP Steam inlet temperature : 365 Degree C

HP Steam Turbine outlet pressure 23 bar

HP Steam Turbine outlet Temperature 220 Degree C

There is 1 extraction of steam from the turbine to HP Heater

2

HP Steam inlet temperature : 365 Degree C

Can somebody suggest the role of pressure in steam turbine. Will steam turbine able to work properly even at low inlet pressure?

I am estimating a future problem while still in erection phase for 60 MW steam turbine. I may need your help to estimate a solution.

Due to some reason my process heater is not able to generate steam at the desired pressure (other parameters are remain same). Therefore I want to know the effect of

**reduce inlet pressure in the HP Steam turbine output**. Th steam parameters are mention belowSteam Turbine is Superheated, Condensing type with Reheater (HP-LP combination)

1.

**Normal condition**Manufacturer : GE

**HP Steam inlet pressure : 98 bar**HP Steam inlet temperature : 365 Degree C

HP Steam Turbine outlet pressure 23 bar

HP Steam Turbine outlet Temperature 220 Degree C

There is 1 extraction of steam from the turbine to HP Heater

2

**Revised condition****HP Steam inlet pressure : 75 bar**HP Steam inlet temperature : 365 Degree C

Can somebody suggest the role of pressure in steam turbine. Will steam turbine able to work properly even at low inlet pressure?

## RE: Steam Turbine performance with variation in inlet steam pressure

## RE: Steam Turbine performance with variation in inlet steam pressure

To make fast estimations you may need some simple steam cycle (heat balance) calculation tool.Check Google.

Good luck

## RE: Steam Turbine performance with variation in inlet steam pressure

because my temperature remains same.Therefore actually the inlet enthalpy will increase little bit. Theoretically the power output might be same?

But what about the kinetic energy turbine needed for driving force. As I understand the kinetic energy will increase with decrease in inlet pressure.

my question is remain same..what will be the effect on turbine if there is reduction in inlet pressure if rest of the parameters including mass flow rate and temperature will same.than what is the role of inlet pressure? will it effect the HP turbine exhaust pressure. as HP Turbine exhaust pressure is not driven by any force (like in LP turbine).

Plz suggest.

## RE: Steam Turbine performance with variation in inlet steam pressure

The real problem is Mass Flow Rate (MFR).

I am not clear are you about new turbine design or drop pressure on existing turbine.

For 1st case you can design new turbine with 75 bar inlet pressure that provides same 60 MW output power for sure.

If you are about existing turbine you can estimate an effect of pressure drop using “turbine constant”

Ct= M*sqrt(Tk/(Pin^2-Pout^2)).

M- mass flow rate in kg/sec

Pin – inlet pressure of component in bar

Pout – outlet pressure of component in bar

Tk temperature on component inlet in K

Very rough estimation for Pin= 98 bar. Pout=23 bar, Tin= 365C, eff=0.9 gives Ct=58.647 and MFR= =221.18 kg/s

When we’d use this Ct (i.e same turbine ) wit Pin=75bar we’ll get MFR = 165.7 kg/s and N= 39.5 MW

This figures were obtained in heat balance calculation tool so the effects on kinematic, losses, etc. were not taken into account. To do this it is necessary to know flow path and cascades geometry of your turbine

## RE: Steam Turbine performance with variation in inlet steam pressure

## RE: Steam Turbine performance with variation in inlet steam pressure

Could you please tell me more about the Turbine constant. I am not aware of it. Also how did you come to a number 221.8 kg/s and 39.5 MW? as I mention it is a HP-LP combination turbine. The total mass flow rate at the inlet of the turbine is 70 kg/s.

racookpe1978

Could you please elaborate how the last stage of LP turbine is effected by moisture?

## RE: Steam Turbine performance with variation in inlet steam pressure

2 racookpe1978It is funny but process simulation shows that 75-bar-cycle (green line) lays completely outside wet area so it looks more safe from wet erosion point.

2 icecoolTurbine constant is derivative from "Stodola Cone Law". It is simple correlation between inlet temperature, pressure drop and mass flow rate.It well suited for 1-stage but can be also applied for raw estimation of multistage turbine.

MFR of 221 kg/s I get in assumption that the section with pressure drop 98-23 bar produced 60 MW of power.

Taking in account MFR of 70 kg/s the power of section at 98 bar will be ~18.99 MW and turbine constant 18.56 . Using this constant value for 75 bar we'll get MFR 52.44 kg/s and power 12.49 MW. See cycles pictures below.

## RE: Steam Turbine performance with variation in inlet steam pressure

Few more questions:

1. I know the turbine upstream pressure. But will the HP Turbine downstream pressure will remain same ( I mean the HPT exhaust pressure is 23 bar when the HPT inlet pressure was 98 bar. Now what happens when the inlet pressure goes down to 75 bar? I know the LPT exhaust pressure is defined by the condenser but what about the HPT)

2. Is the 52.45 kg/s is the max flow at 75 bar condition? if not then how to calculate the choked flow?

Probably I am going more theoretical. But, I am thinking from every possible way.

## RE: Steam Turbine performance with variation in inlet steam pressure

Other way is to try to remodel your turbine with preliminary design tool.

What you know about it? Is HP section governing (Curtis) stage? Are there reheater after HP section? Are there any other reheaters? How much and at what pressure steam is extracted to deaerator and feedwater heaters?

Do you know model number of this GE turbine?

## RE: Steam Turbine performance with variation in inlet steam pressure

If turbine has MFR=70 kg/s at design point , why you expect choke at 52.4 kg/s ?

## RE: Steam Turbine performance with variation in inlet steam pressure

If the guaranteed capacity at 98 bar is 70.0 kg/s, the turbine manufacturer has probably included some flow margin in the design (typically, 2% to 3%). In any event, the maximum capacity at the 75 bar will be the maximum capacity at 98 bar multiplied by 0.742.

I suspect that the generator electrical output at the lower inlet pressure will also be lower. As a first approximation, I would assume that the efficiency of the HP turbine remains constant. If the enthalpy drop through the HP turbine remains constant, the electrical output will be less due to the reduction in flow capacity.

It is difficult to predict the effects on all the equipment in the cycle (boiler including reheater, feedwater heater, condenser, generator, etc.) without the aid of a performance program like GateCycle or PEPSE.

Best of luck!

## RE: Steam Turbine performance with variation in inlet steam pressure

-accept the 25% reduction in STG load and mass flowrate, or

- buy a larger steam turbine designed for the new design conditions, or

-revise the upstream process to deliver the originally specified 98 bar steam pressure (and find out how this mistake was not found earlier), or

-add a gearbox and run the STG at another RPM- not a likely scenario

"Whom the gods would destroy, they first make mad "

## RE: Steam Turbine performance with variation in inlet steam pressure

The HP turbine exhaust is connected to moisture separator which is connected to reheater. After reheating the steam enters inside the LP Turbine. HP Heater 1 extraction is at 36 bar, 256 degree C, and flow is 5.2 kg/s. The Deaerator and other HP heater take extraction from the HPT exhaust.

Choking depends on upstream pressure only if the ratio with downstream is approx more than 2. Thats why I was thinking of any possibility of increasing mass flow rate. But choking might not be practical in HP turbine?

Stgrme, davefitz ,

Thanks for your input. Your calculation of power output and calculation with equation suggested by Hermit47 matches very closely.

I understand there is no possibility other than increasing the pressure for getting more power out.

Could you suggest what dictates the Exhaust pressure of a turbine. Also is there any way of calculating it theoretically.

Thanks again for all your suggestions.

## RE: Steam Turbine performance with variation in inlet steam pressure

Is this turbine of condensing type or back-pressure one?

## RE: Steam Turbine performance with variation in inlet steam pressure

## RE: Steam Turbine performance with variation in inlet steam pressure

I U'd give me temp after reheater and data about other extractio I can remodel thi turbine. Are theer any references to its original specification?

## RE: Steam Turbine performance with variation in inlet steam pressure

Please see the attached HMBD for 60MW case.

## RE: Steam Turbine performance with variation in inlet steam pressure

If we consider the ACC as a "compact heat exchanger" then a simplified view would be that e= effectiveness = 1-exp(-NTU), which works for the special case of one fluid ( condensing steam) apparently having an infinite Cp. NTU= U*A/(W*Cp), air,, where W= mass flow of air over the fins and Cp= air heat capacity.

e= effectiveness = (Tao-Tai)/(Tss-Tai)

Tai= ambient air temp

Tao= air outlet temp, determined via Q= W*Cp,a*(Tao-Tai)

Tss= steam saturation temperature.

These equations are sufficient to determine the back pressure ( ie saturation pressure) if one knows Wa from the fan curves. If the curves are no available, one can estimate the value of e from the vendor supplied performance predictions.

"Whom the gods would destroy, they first make mad "

## RE: Steam Turbine performance with variation in inlet steam pressure

See comparison of pressure distribution along flow path for Pin 98 and 75 bar

## RE: Steam Turbine performance with variation in inlet steam pressure

What is component number? Does it relates to number of stages in a turbine.

Also it seems to me that the pressure lines are parallel to each other in most part of the expansion. That means apart from low pressure region the expansion at different pressure will be same?

## RE: Steam Turbine performance with variation in inlet steam pressure

I run meanline analysis in AxSTREAM. Here component stands for either rotor or stator.

The expansion is almost the same in both cases excluding last stage (see charts below)

And the reaction of last stage will be negative. It means that rotor works in compressor mode.

All simulation above where performed on sample turbine obtained via preliminary design.

Also it was meanline analysis only.

For more reliable results it is necessary to use actual flow path geometry and streamline analysis. It is much more expensive work.