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How to Calculate Steam Consumption During Plant Start Up

What is Steam Consumption in a Pipe?

Steam produced and steam utilized at the point of application is not same. Normally steam produced/generated in boiler is more than what is required to be utilized at the point of application.
The difference in steam produced and utilized is on account of following reason:
  • Steam condensation when it travels on its way to point of use mainly due to exposed surfaces.
  • Leakages (if any)
When steam condenses on the wall of the exposed/uninsulated steam pipe then it gives up its enthalpy of evaporation. Proper utilisation of steam and its optimal use helps in saving in the input cost of water and coal. Saving of each kg of steam is directly proportional to the saving of some percentage of water, coal and electricity.

Calculation of steam consumption in a pipe during the start-up operation and normal continuous operation is possible as per the equation---1 given below.

Steam Consumption in Steam Mains

Steam consumption within the pipe system needs to be judiciously monitored and controlled. The rate of condensation of the steam in the steam piping network depends upon the type of load i.e warm up load and running load.
Rate of condensation of steam needs to be taken into account for sizing the steam traps and also in finalising the boiler output.

What is Warm-Up Load ?

During the plant start-up after a long time or from cold, then the steam is required to heat the system uniformly to bring the system close to the normal working temperature of the system.
Warm up load is the steam load which is associated with the steam consumption during the start-up of the plant from the cold shut down or start up after a very long time. Rate of condensation of steam during the warm-up period is maximum. Design of steam trap shall be based on this load.
Good practice is to warm-up the system very slowly for safety-reasons, pipes have the benefit of reduced thermal and mechanical stress. Thus results in following benefits:
  • Eliminations of leaks.
  • Lower-maintenance-costs.
  • Longer life for the pipes.
  • No water hammer.

What is Process Plant Running Load?

Process plant running load is the steam load which is related with the normal (full load) continuous load of the plant. Rate of condensation of steam during the full load running load of the plant is minimum.

How to Warm-Up a System?

Refer figure
Uniform and slow system warm-up can be achieved by a small by-pass valve in parallel to the main line isolation valve.
Time required to warm up the pipe network decides the warm-up (bypass) valve size. This valve can be of manual type or automatic type depending upon the user/client. It is always better to discourage the practice of using main valve for warm-up instead of by-pass valve. Since main valve is far bigger in size (designed for full flow requirement) and not appropriate to use for small flow during the warm-up period.
As shown in the figure:1, before the main valve /by pass valve seperator is installed to ensure that steam passing through the valve should be dry to protect the valve wear and tear.
I we provide the sufficient time for the warming-up, then it is possible to get the following benefits:
  • For minimising the pipe stress
  • For operational safety
  • Reduce start-up loads on boiler
The steam flowrate required to bring a pipework system upto operating temperature is a function of:
  • Mass
  • Specific heat of the material
  • Temperature increase
  • Enthalpy of evaporation of steam or Enthalpy of saturated steam
  • Allowable time
Where,
ms: Mean rate of condensation of steam in kg/hr
W: Total weight of pipe plus flanges and fittings in kg
Ts: Steam temperatureoC
Tamb: Ambient temperature
Cp: Specific heat of pipe material in kj/kgoC
hfg: Enthalpy of evaporation or saturated enthalpy at operating pressure in kj/kg
t: Time of warming-up in minutes
Note : Suppose if the warm-up/heating time of the system is increased to 12 minutes from 6 minutes, then the initial flow rate of the steam can easily be reduced to half. If the warm-up/heating time of the system is increased to say 24 minutes, then the initial steam flow rate for heating can be reduced further.
Method to calculate the Warm-up condensing load for a period of 30 minutes in a steam system
Let,
The system consists of carbon steel pipe of size ---- 150 mm
Steam Pressure ---- 16 bar(g)
Steam temperature (Ts) ---- 201oC
Entalpy of evaporation corresponding to above parameters hfg ---- 1933
Ambient air temperature (Tamb) ---- 20oC
Length of the pipe shall be ---- 120 m
Cp of steel is ---- 0.49 kj/kgo C
Warming-up time in minutes (t) ---- 30
Total weight of pipe plus flanges and fittings in kg (W)
Pipe network consists of 10 pairs of PN-40 flanged joints and two isolation valves
Table1: Typical weights of steel pipe, flanges and bolts, and isolating valves in kg.
Pipe size(mm)Sch.40 pipe kg/mFlange weight per pairIsolating valve flanged PN40
PN40ANSI150ANSI300
15028.228.026.03288
To find W, find the mass of various steam main items from Table 1
150 mm steel main = 28.2 kg/m
150 mm flanges to PN-40 = 28 kg per pair
150 mm stop valve = 88 kg each
Rate of condensate generated during warm-up time is given by

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