bthu-thermex.pl | method DM3

DM3 method
The DM3 method consists in performing strain gauge measurements of loads in hanger rods and their mutual interactions with stepwise progressive complete loosening of the rods. Such a procedure is very tedious and in many cases difficult and sometimes even impossible to perform. Therefore, we have developed a modified DM3 v.1 method, consisting in measuring the loads in rods using a hydraulic force gauge with a strain gauge correction of its indications and the simultaneous determination of mutual interactions in rods. These actions are measured up to ten adjacent rods after a given load change (e.g. partial loosening) in one of the rods (Fig. 1).

Fig. 1. Scheme of the measuring system.

Fig. 2. Stress concentration resulting from overload and underload of boiler screen suspensions.

The strain gauge correction of the hydraulic force gauges must be taken into account in non-free systems and allows to maintain the accuracy of the DM3 method adjustment with a significant reduction in the time of work. This modification is especially useful on old units with corroded threads and difficult access to them. We know from the present experience that the value of this correction ranges from 0 to even 30% of the measured value of the ‘hydraulic’ force. The size of the correction depends on the diameter of the rods and the method of their installation (if the the rods are rigid or with more or less flexible springs), the type of suspended load (header or hanger pipe, airtight waterwall, steel coils). The measurements carried out in this way allow to determine the actual forces acting in the rods along with their mutual interactions taking into account changes in the stiffness of the supporting structure of the object and the object itself. This procedure leads to the definition of the influencing numbers kni and the actual correction factor k which is the quotient of the sum of the measured loads and the sum of the designed loads important in further numerical calculations.

kni = Pni/Pnn

kni – influencing number for the i-rod determined by the load change in the n-rod,

Pni – load change in the i-rod caused by the change in load on the n-rod,

Pnn – force value in a loosened rod.

Influence numbers determined in this way makes possible to determine the value of the force in any rod after any load change in adjacent rods. Using the matrix notation, it is possible to present in the general form the relationship between the loads in the hangers along the entire boiler contour:

where:

{S} = {P} + [Mw]{w}

{w} – ΔPn{1/Pnn}

[Mw] – influence matrix,

ΔPn – load change in the n-rod,

{P} – vector of real forces before starting the adjustment

{S} – vector of forces in the rods after a load change by ΔPn

Calculations of the magnitude of load changes which are necessary to be carried out in individual rods are performed on a computer, using our own special software that enables a computer simulation of the load adjustment process in all rods working interdependently (Fig. 2).

This technology allows for the load distribution in the rods proportionally to the design assumptions, eliminating excessive stress concentrations in the waterwalls and structural nodes and maintaining the unchanged geometric position of the controlled object. The unchanged position is very important in the case of an object consisting of several independent systems connected with each other by compensators (e.g. fluidized bed boilers, etc.).