Anti-surge protection system for compressors - III
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:
- To monitor online, any peaks on journal bearing in radial directions and making center line tracing (orbitals) to check if there is unbalance or misalignment or mechanical looseness present instantly after starting operation, early detection is better.
If these forces are present, it generates an excitation force that increases the peaks of vibrations to dangerous values.
Axial displacement probes (thrust):
- To monitor axial movement of rotor during operation, as excess movement of rotor can cause rupture contact between rotating and stationary parts of compressor that can cause complete damage and explosion of compressor.
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:
- To monitor only and measure overall vibration whole train running to check if there is any looseness or any strain in pipes affecting the alignment with small peaks also to detect if any extra forces as unbalance and misalignment are present.
It can use vibration probes to measure its reading with no installed new probes.
Speed controller
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.
Inputs:
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):
- To monitor position of control valve (CV) plugs during operation and startup; whether it is stuck or not.
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):
- To check if steam entering turbine is superheated or not as saturated steam can cause erosion for turbine blades and damage it, also to calculate enthalpy of inlet steam.
Wheel chamber pressure (WCP):
- To measure pressure difference of steam across nozzles to monitor their performance, if they are affected by internal erosion that can cause poor pressure to velocity conversion inside it.
Steam pressure (Pst):
- To check if steam entering turbine is superheated or not; also to calculate enthalpy of inlet steam.
Steam flow (Fst):
- To calculate the power generated by turbine, to calculate efficiency.
Exhaust (pressure and temperature):
- To monitor steam coming out of turbine to know if there is a back pressure from condenser to turbine, also to check that all turbine stages are efficiently expanding steam, if there are any of the previous reasons, therefore turbine efficiency will be affected badly.
It is used to calculate exhaust steam enthalpy and calculate efficiency.
Over speed and speed sensors:
- To measure turbine speed and they are three sensors and speed controller compare the reading from the three sensors and their average, if two of three sensors are damaged, trip for machine will take place.
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
- To compensate compressor data with turbine loads and speed in order to achieve optimum performance for the whole train.
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
- To be mounted with speed controller in order to decrease loads at cases of high excitation force resulting from unbalance and misalignment.
Outputs
Calculation results (y) to (E/P) as an input for it.
I/H:
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:
- Boiler steam drum pressure in order to check steam availability in system before passing critical speed ranges.
- Compensator, surge control unit, vibration control unit are all mounted with the speed controller as an input.
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.)
Anti-surge protection system for compressors - I
