REFINEMENTS IN THE SPECIFICATION OF
ALLOWABLE FORCES AND MOMENTS FOR POWER PLANT PRESSURE BOUNDARIES for Boilers
and Turbines
S. Soundrapandian, B. E., M.
Tech., M. I. E. (I), Chartered Engineer (I),
No. 110 (New No. 192), Mannar Samy Koil Street, Vasant Apartments, Flat
No. 4A, I – Floor, Royapuram, CHENNAI – 600 013, Tamilnadu State, India.
E-mail: sspenggconsultant@yahoo.com
Power plant pressure boundaries are designed to withstand different loads. These loads can be classified as primary loads, secondary loads and occasional loads. The present practice in the design of boilers and turbines used in the power plants is to design them for primary loads and check them for secondary loads and occasional loads. As the design of any product is an evolution, this requires several iterations. In order to reduce the cycle time in design, the boiler designer and the turbine designer specify two sets of allowable forces and moments that can be permitted on their respective equipment from the connected piping. After these allowable forces and moments are specified and agreed, the boiler designer, the turbine designer and the piping designer proceed independently to finalize their respective designs. This speeds-up the project execution and is a part of the “Concurrent Engineering”. The present practice is to specify a set of six numbers (three forces in three mutually perpendicular directions and three moments in these three directions). These numbers are used to check the acceptability of the forces and moments imposed by the connected piping on the respective equipment. The present practice is to limit the combination of all the forces and moments due to primary loads, secondary loads and occasional loads to one set of six numbers. This practice is approximate and requires refinement. In the present work, a method is given, which properly addresses the limits of the forces and moments due to various classes of loads. Three sets of six values each is given, that can be used for evaluating the acceptability of the various forces and moments. Rules for combining the allowable forces and moments due to various classes of loads are also given.
The power plant pressure boundaries of various make are ready-made or custom-built. In either case, the connected piping imposes forces and moments on them. It is a standard practice to specify these values to enable the piping designers to carryout their work. As the present practice is empirical in nature, a scientifically developed method is proposed to address this problem.
From time immemorial, power plant pressure boundaries are made to withstand different loads. With the advent of the industrial revolution, the requirement of specialization and integration came into existence. Hence, interfacing of various activities has become a pre-requisite. In the past century, the concept of allowable forces and moments on the power plant pressure boundaries has been introduced. Historically, the power plant pressure boundaries were designed only for the primary loads. With the progress of knowledge, secondary loads and occasional loads are also considered in the design. Hence, a requirement for specifying the allowable forces and moments on the equipment due to the secondary loads and occasional loads has arisen.
The present problem is to evolve a methodology to calculate the allowable forces and moments, scientifically. The availability of the modern computers with high speed and huge memory has made this possible.
The legacy method is to specify empirically, one set of six numbers (forces in three mutually perpendicular directions and moments in these three directions) that can be allowed.
The method proposed specifies three sets of six numbers each, giving the values of allowable forces and moments due to primary loads, secondary loads and occasional loads. A set of rules to combine these is also given.
The equipment is designed using conventional methods. Methods, used by codes for treatment of allowable stresses for various classes of loads is used to calculate the allowable loads due to primary loads, secondary loads, occasional loads, primary loads + occasional loads and primary loads + secondary loads are provided.
This procedure can be used for the boilers, turbines, compressors, condensers, pumps, valves, heat exchangers, pressure vessels, bunkers, silos and storage tanks.
The proposed method is based on several assumptions applicable to the mechanical static equipment, designed using the equivalent static methods. As equipment in the critical areas like nuclear industries, aero industries, space industries and defense industries are designed using dynamic loads and transient loads, the proposed method is not applicable to these industries.
In critical applications, a real life simulation, considering elastic-plastic behavior, non-linearity, transient and dynamic behavior is used. This requires the material behavior after yielding, loading time history, creep behavior of the material, fatigue behavior of the material and creep-fatigue interaction rules.
Example of Calculation for a Boiler
The pressure boundaries of a 500 MW boiler is taken as an example. The interface between the boiler headers and the connected external piping is considered. In the large boilers, the following components interface with the connected external piping: (1) economizer inlet header, (2) main steam outlet header, (3) re-heater inlet header and (4) re-heater outlet header. The typical values for a 500 MW boiler are given below:
Table: Typical Values of the Allowable Forces and Moments for a 500 MW
Boiler Headers
|
Header description |
Number of headers per boiler |
Number of inlets/ outlets per header |
Fx (kN) |
Fy (kN) |
Fz (kN) |
Mx (kN-m) |
My (kN-m) |
Mz (kN-m) |
|
Economizer inlet header |
1 |
1 |
30 |
30 |
30 |
50 |
50 |
50 |
|
Main steam outlet header |
1 |
2 |
50 |
50 |
50 |
100 |
100 |
100 |
|
Re-heater inlet header |
1 |
2 |
40 |
40 |
40 |
80 |
80 |
80 |
|
Re-heater outlet heater |
1 |
2 |
60 |
60 |
60 |
120 |
120 |
120 |
Where the number of inlets/ outlets per header is two, the allowable forces and moments per inlet/ outlet are taken as 60% of the values allowed per the respective header. The boiler pressure boundaries are strain insensitive. The turbine pressure boundaries are strain sensitive. Hence, the values of the allowable forces and moments specified for the connected 500 MW turbine are lower than the above values. This method of specifying the allowable forces and moments for the boiler pressure boundaries is about fifty years old. As the modern computers were not available in those days, only the forces and moments due to thermal expansion loads were compared. The effect of internal pressure in the boiler external piping, the effect of the hydraulic test pressure, the effect of the weight of the water used for the hydraulic test, the self weight of the components, the effect of the wind forces, the effect of the earthquake loads, the effect of water hammer, the effect of steam hammer and the effect of the safety valve blowing jet reaction are not considered. Out of the above loads, the following constitute the primary load: internal pressure, self-weight of components and the weight of cold water during hydraulic test. The following constitute secondary load: thermal expansion load and the loads due to foundation relative settlement. The following constitute occasional load: wind load, earthquake load, water hammer load, steam hammer load, safety valve blowing jet reaction and hydraulic test pressure load. Out of the primary loads, the internal pressure in the piping is self-limiting. Hence, this is not compared with the specified allowable forces and moments. In evaluating the secondary loads, only the range of forces and moments are considered. The absolute values are not considered, since the boiler pressure boundaries are not sensitive to them. Six occasional loads are indicated above. The normal practice is to assume that these six occasional loads are mutually exclusive. The effect of cold springing of the boiler external piping is not considered. This is due to the fact that the boiler pressure boundaries are insensitive to the cold springing. For the sake of conservatism, the cold modulus of elasticity of the boiler external piping is used in the stress analysis of the same. The values of the forces and moments are the range of values, and hence, no sign is assigned to them. In evaluating the acceptability of the actual (calculated) forces and moments, the reserve available in few values is considered to offset the excess values of few forces and moments. The foregoing indicates that there is lot of scope for improving the methodology adopted in the legacy method. The following gives a recommendation to improve the legacy method:
1. Allowable force due to primary load = (Sh / (1.25 Sc + 0.25 Sh)) x value from Table
2. Allowable moment due to primary load = (Sh / (1.25 Sc + 0.25 Sh)) x value from Table
3. Allowable force due to occasional load = 0.2 Sh / (1.25 Sc + 0.25 Sh) x value from Table
4. Allowable moment due to occasional load = 0.2 Sh / (1.25 Sc + 0.25 Sh)) x value from Table
5. Allowable force due to secondary load = value from Table
6. Allowable moment due to secondary load = value from Table
Where,
Sc = Allowable stress for header material at ambient temperature, MPa
Sh = Allowable stress for header material at header metal working temperature, MPa
The following gives rules for combining various loads:
a. Allowable force due to primary load plus occasional loads = 1.2 x Value from (1), above
b. Allowable moment due to primary load plus occasional loads = 1.2 x Value from (2), above
c. Allowable force due to primary load plus secondary loads = Value from (1) plus value from (5), above
d. Allowable moment due to primary load plus secondary loads = Value from (2) plus value from (6), above
The foregoing
is in line with the rules of ASME B31.1 (Code for Power Piping). The rules for
combining the secondary forces and moments with the occasional loads require
more research, and hence, are not given here.
As the forces and moments are only intermediate values, the concept of allowable forces and moments is likely to become obsolete after few decades in many industries. A computer simulation of the behavior of the equipments under various forces and moments is likely to replace the concept of allowable forces and moments in many industries.
The concept of allowable forces needs refinement. In the long run, the concept of allowable forces and moments is likely to become obsolete in many industries.
1. ASME B31.1 – “Power Piping”, 2004
2. ASME Boiler & Pressure Vessel Codes: Section I – “Power Boilers”, 2003
3. ASME Boiler & Pressure Vessel Codes: Section VIII, Division 2 – “Pressure
Vessels – Alternate Rules”, 2003