Load change

In fire-tube furnaces developed in the nineteenth century, such as typified by the Scotch-Marine boiler (Fig. 1), thin currents of water contact a multiplicity of tubes thus, the hot gases transmit heat simultaneously to aH regions of the bulk of the water. Therefore, this boHet—furnace combination steams readily and responds promptly to load changes, and is, for a given amount of heating surface, the least expensive of aH furnace—boHet instaHations... 

FIG. 8-33 (a) Comparison of FF (steady state model) and PID FB control for load change (h) comparison of FF (dynamic model) and combined FF/FB... 

The speed control operates the governing valve to maintain steam flow commensurate with load demand while holding speed essentially constant. For sudden load changes there will be a short-time overshoot, and a special case is the instantaneous loss of load, load dump at full load. The usual specification states that the overshoot on load dump must not exceed 9 to 10 percent of rated speed. The settled speed rise will of course be equal to the regulation, 4 or 6 percent for a NEMA Class C or B governor and less than I percent for Class D. 

From what we have said already, we can write down an energy balance which must be met if the crack is to advance, and fast fracture is to occur. Suppose a crack of length fl in a material of thickness t advances by 8a, then we require that work done by loads > change of elastic energy + energy absorbed at the crack tip, i.e. 

Load pattern The load change over time. 

Figure 6-31 illustrates control schemes for the single stage unit which allow greater stability in performance. As the load changes for a fixed suction pressure, the process fluid is replaced by an artificial load (usually air Figure 6-31, item 1) to maintain constant ejector operation. An artificial pressure drop can be imposed by valve (2), although this is not a preferred scheme. Wlten the addi-... 

The calihration curve (three-phase induction motor) of Figure 14-5 is essential to determine the load change based on the current change for motors helow 200 hp or with load variations of more than 5-10%. 

The turbine manufacturer should be given any information regarding partial loads, change in loads and speeds, speed range, changes in steam pressure (not just variations)... 

Turbulent or unbalanced media flow (i.e., aerodynamic or hydraulic instability) does not have the same quadratic impacts on the vibration profile as that of load change, but it increases the overall vibration energy. This generates a unique profile that can be used to quantify the level of instability present in the machine. The profile generated by unbalanced flow is visible at the vane or blade-pass frequency of the rotating element. In addition, the profile shows a marked increase in the random noise generated by the flow of gas or liquid through the machine. 

Other forces, such as the side-load created by V-belt drives, also generate unique frequencies or modify existing component frequencies. For example, excessive belt tension increases the side-load on the machine-train s shafts. This increase in side-load changes the load zone in the machine s bearings. The result of this change is a marked increase in the amplitude at the outer-race rotational frequency of the bearings. 

Most of the microprocessor-based instruments are designed to handle steady-state vibration data. Few have the ability to reliably capture transient events such as rapid speed or load changes. As a result, their use is limited in situations where these occur. 

Actual running speed remains relatively constant on most generators. While load changes create slight variations in actual speed, the change in speed is minor. Generally, a narrowband window with lower and upper limits of 10 per cent of design speed is sufficient. 

The reason for speed variations in constant-speed machinery is variation of process load. For example, a centrifugal pump s load will vary due to the viscosity of the fluid being pumped or changes in suction or discharge pressure. The pump speed will change because of these load changes. 

Constant versus variable load As with constant-speed machines, true constant-load machines are rare. For the few that may be specified as having constant load, there are factors that cause load changes to occur. These factors include variations in product, operating conditions, and ambient environment. These variations will have a direct, and often dramatic, impact on a machine s vibration profile. 

Figure 44.33 Trends must be adjusted or normalized for load changes...

Periodic boiler restarts or frequent load changes that lead to boiler cycling this inevitably produces carryover problems... 

Routine work typically includes monitoring and reporting load changes, unusual occurrences, machinery failure, steam leaks, and the like. In addition, boiler plant personnel carry out a number of other specific functions, including those tasks described in the following sections. 

---