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Measurement and load adjustment of the boiler hangers
Power boilers are in general a hanging type mechanical systems, fixed on a rigid or flexible hangers. Rod – Spring Box – Main nut system forms the hanger. To avoid excessive stress concentration in the waterwalls and other boiler components and to ensure their correct positioning, the loads in the hangers have to be adapted to the design load distribution.

Measurement of loads (forces, kN) in hangers is performed with the use of hydraulic cylinders ensuring axial relief of the main nuts fastening the rods (Fig. 1). The force acting in each hanger is calculated on the basis of the measured hydraulic pressure and the working surface of the piston cylinder used in the relieving steel element (loosening device). In particular situations, an electronic force sensor is attached to the measuring system, ensuring an accuracy below 1%. The accuracy of the pressure gauge used to measure the hydraulic pressure is ± 5 bar. The measuring system eliminates the torque used to loosen the nut securing the hanger rod. It is not allowed to measure the load using tools causing the rod twist.

The accuracy of the load measurement of the used hydraulic system is periodically checked on a testing machine under laboratory conditions (Fig. 2). The value of the force acting in the rod is determined on the basis of an experimentally determined characteristic describing the relationship between the pressure read from the pressure gauge and the force acting in the measuring component. The system (Fig. 1) consists of a steel mounting element adapted to the diameter of the thread rod and a set of hydraulic cylinder appropriately selected for the expected load occurring in the rod, together with a hydraulic pump and pressure gauge.

The pressure at which the nut is being loosened creates a force in the measuring steel component that balances the force occurring in the rod. This value is close to the actual force value in the rod. The real value of the force acting in the rod can be calculated after determining the appropriate load correction, depending on many factors, e.g. support stiffness of the measuring fixture, grate structure, type of load, etc. (DM3 v1 method).

To adjust the hangers, the needed information is the value of the load of the rod of each hanger.

Design loads which occurs in hanger rods are usually specified in the documentation – on the technical drawings of the boiler (individual pressure elements of the boiler).

The most common information about loads occurring in the hanger rods is:

  • the dead weight steel, refractory, Inconel, insulation and slings (total waterwalls weight),
  • water filled in waterwall pipes,
  • estimated weight of the ash, slag and other impurities.

Fig 1. An example of a measuring component for adjusting hangers used by BTHU Thermex s.c.
Fig.2. An example of a laboratory stand for determining the accuracy of the load measurement of the hydraulic system used in measuring and adjusting boiler hangers.
Adjustment of the loads
Adjustment of the loads in the hanger rods is carried out in such a way that its values after adjustment of the tested mechanical system assume values proportional to the design loads specified in the current technical documentation of the boiler. Therefore, it is necessary to create a database entered in the measurement card in which the values of design loads and measured forces are given. These values are entered in accordance with the adopted numbering of suspensions marked on the object.

The numbering of hangers covers all testing independent mechanical systems. The selection and division of hangers into independent systems are made on the basis of the boiler documentation and engineering experience, taking into account the susceptibility of hangers of individual boiler components and their mutual interactions.

The adjustment of the loads in the hangers proceeds in accordance with the calculation results obtained from the numerical simulation of the adjustment carried out with the use of the proprietary calculation program. The created adjustment table contains the values of load changes that should be executed in individual hangers in order to obtain the desired load distribution (proportional to the design loads). During the computer simulation, the load correction factor k is determined, which enables the proportional distribution of the measured load of the system into individual hangers in accordance with the assumptions of the boiler design. where:

  • ∑Pn, Measured – the sum of loads measured in hangers before the adjustment (actual weight of the system),
  • ∑Pn, Designed – sum of design loads in accordance with the documentation, taking into account any structural changes (design load),
  • n – number of hangers in the tested mechanical system.

Multiplying the design loads by the correction factor k, the value of the reduced loads to be sought when adjusting the hangers is calculated. The permissible tolerance range of the loads after hanger adjustment is also calculated, assumed to be within ± 10% of the reduced load.

Checking measurements
The verification of the hangers load consists in re-measurement of the loads in all hanger rods of the tested mechanical system. It is necessary to check whether the distribution of loads after regulation does not differ from the calculated reduced values. The maximum deviation of the load after adjustment in relation to the reduced load by ± 10% is allowed.

Securing the hangers after adjustment
The currently used securing of the main hanger nuts in boiler pressure elements are counter-nuts or diamond washers. The counter-nuts are placed on the hanger rods and after the adjustment is completed, tightened (with counter-nuts) to the main nut with a torque of approx. 40 Nm. The diamond washer should be welded with one end to the hanger washer under the main nut (rigid hanger) of or upper plate of the spring box of the hanger and the other end should be bent over the nut. When the hangers has a spring box units, the position of the nuts of the locking safety pins on the spring locking pins should also be checked. Before and after adjustment, the upper plates of the spring unit should be unlocked, i.e. the nuts of the locking safety pins should be spaced from the plate surface by a value of about 10 – 20 mm to ensure free bending / expansion of the springs during start-up, operation and shutdown of the boiler.
Documentation after hanger adjustment
Current documentation of the boiler hangers is necessary to execute the adjustment. On its basis, drawings are prepared – distribution diagrams of independent mechanical systems with their numbering and tables of design loads for hangers.

After adjusting the hangers of the mechanical system (e.g. combustion chamber waterwalls, 2nd pass waterwalls, superheater headers, eco, etc.), a REPORT is created – including the following tables:

  • design loads,
  • loads measured before adjustment,
  • reduced loads,
  • loads measured after adjustment,
  • deviations (%) occurred between the measured loads after regulation and the reduced loads.

The adjustment results contained in the above-mentioned tables are presented in the form of clear graphs showing the load distribution before and after the hanger adjustment.

The report contains details of the process of measuring and adjusting loads in hangers, conclusions and recommendations for further operation of the measured systems.