OR WAIT null SECS
In the second part of this series, the author highlighted the uses of ASCS and discussed multi-point and mono-point systems. This article focuses on the vibration control unit, speed controller and surge controller unit.
Vibration control unit
Radial bearing vibrations:
If these forces are present, it generates an excitation force that increases the peaks of vibrations to dangerous values.
Axial displacement probes (thrust):
Critical speed checking:
Installing critical speed ranges of compressor to be mounted with speed controller of prime mover in order to prevent running of compressor at that speed range.
In cases of turbines operating compressors, the turbine rotor has critical speed range and the compressor has critical speed range so the speed controller is adjusted on the speed range of the whole train.
There are different types of critical speeds, lateral critical speed (for impeller spacers or sleeves), torsional critical speed (impeller slipping on rotor), and radial critical speed (resonance).
Vibration frequency analyzer:
It can use vibration probes to measure its reading with no installed new probes.
It is used to control speed and loads of compressor prime mover, let’s talk about condensing steam turbine driving a multi-stage centrifugal compressor as an example.
The speed controller should have some inputs and outputs in order to operate accurately.
Turbine load curves:
Constant load curves made by the manufacturer after turbine testing, it contains ranges of critical speeds, load curves which are responsible for raising loads of turbine by safe temperature rising step to not damage internal parts of turbine (blades, blade carriers) with rapid overheating and over stress.
Turbine stress sensor (TSS):
It is a sensor that senses the turbine thermal stresses during startup and operation for huge steam turbines (500MW and over) , it measures the thermal stresses on steam turbine casing and record it , if it passes over a certain designed value , it trips the turbine immediately for turbine safety .
Control valve stroke (L):
Monitoring valve springs to check if valve is closing 100% or there is a leak through from it.
Leak through from CV can cause the turbine to run with speed up to1000 rpm instantaneously after opening emergency stop valve.
Its main benefit is to transfer turbine running load in a function of valve stroke.
Steam temperature (Tst):
Wheel chamber pressure (WCP):
Steam pressure (Pst):
Steam flow (Fst):
Exhaust (pressure and temperature):
It is used to calculate exhaust steam enthalpy and calculate efficiency.
Over speed and speed sensors:
Over speed sensors are also three sensors detect over speed then trip for machine (2 of 3), speed sensors readings should be connected to speed controller.
Surge control unit
Operate compressor at an operating point a distance from surge line.
Input data manually by operators (X):
Data input by operators in cases of problems to operate manually (steam problems).
Note: input to speed controller (x).
Vibration control unit
Calculation results (y) to (E/P) as an input for it.
It is electric/pressure signal converter as oil enters with control oil pressure and leaves with a pressure between 2-4 bars according to the electric input signal to it from speed controller.
The input signal to it from speed controller is an analog signal 4-20 mA, now it is modified to digital ones.
Pressure signal coming out to the actuator of CV by which moves CV plugs to change valve stroke to change load of turbine according to speed controller signal.
New edition of speed controllers have automatic operating modes that can operate turbines according to load curves and critical speed, but the new system is to add steam inlet conditions: pressure, temperature, steam flow to ensure safe operation during running and raising load, can also add the following:
From the outputs of speed controller is the efficiency calculator that calculates the steam turbine efficiency online by means of input data of inlet /outlet steam conditions, to monitor steam turbine performance and observe any deviation in its performance and to analyze and troubleshoot problems early.
Steam tables may be installed in the speed controller as a library to facilitate enthalpy calculation for efficiency and power calculation.
(The author is Turbomachinery Manager at Abu-Qir Fertilizers Company in Alexandria, Egypt. He has a Masters Degree in Mechanical Engineering.)