ASHRAE Journal - August 2019 - 24

TECHNICAL FEATURE

24

ASHRAE JOURNAL

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A U G U S T 2 0 19

FIGURE 2 Efficiency comparison before and after controls upgrade.

Existing Efficiency Curve

Steam Consumption (lbs/ton·h)

25

Projected Efficiency Curve

20
15
10
5
0
0

500

1,000

1,500
2,000
Load (Tons)

2,500

3,000

3,500

FIGURE 3 Graph of surge line operation.

Minimum Vane
Calculation Line

Hot G

as Op

erati o

n Zon

e

Surge Line
Pressure Ratio

water makeup in cooling tower condenser water systems
and other uses.
The original steam turbine chiller governor panels,
and often the replacement panels, are not generally
suited to precise changes in turbine speed or inlet guide
vane adjustment and offer only limited system monitoring and feedback. The three options that typically are
used to improve performance, along with normal maintenance and upgrades, include:
* Use of an upgraded steam panel and control system;
* Installation of a new single-stage driveline; and
* Hybrid installation with an electric driveline.
Improving the performance of steam turbines allows
them to do the following:
* Optimize turbine speed and inlet guide vane operation, while minimizing hot gas bypass;
* Allow varying and colder off-peak condenser water
temperature for improved efficiency;
* Reduce condenser water pump operation, especially off-peak;
* More closely approach surge points without use of
inefficient methods such as manual hot gas bypass control or other inefficient strategies;
* Enhance performance with the latest single-stage
compressors;
* Incorporate automation of the surface condenser
and hand valves;
* Increase turbine power management through noncontact torque telemetry; and
* Install a coupled electric drive for off-peak use.
Figure 2 shows the performance of steam consumption in a typical application before and after a controls
upgrade. Data is based on hourly steam consumption
data as provided by the utility real time meters and
building cooling load based on data logged by building
staff.
Recommended features incorporated into a control
panel and system include the following:
* Measurement of condenser/chilled water temperatures/flow rates and approaches;
* Refrigerant/oil pressures and temperatures;
* Steam pressures/flow rates at various points;
* Steam consumption (lbs/ton);
* Horsepower and torque;
* Automation of surface condenser, including condensate level, alarms, pumps, vacuum, and control
valves; and

Process Load Zone

"Smart" Minimum Vane
Calculation Line: Generated
From Machine Learning

Volume Flow

* Automated and correctly sized hot gas bypass valve.

Upgrading Turbine Control Systems
The challenge of safely operating a steam turbine during off-peak operation involves prudent and careful
procedures, since running into a surge condition can
cause costly damage to the turbine system. Employing
an upgraded control system, control of the turbine
speed can be fully integrated into the control system
programmable logic controller (PLC). A speed sensor
wired directly to the PLC is typically used for speed feedback with the internal proportional integral derivative
(PID) control loop regulating the actuation of the turbine
governor valve to maintain the desired speed. Control of
the turbine speed is maintained by the PLC, including
slow roll, ramping to rated speed and over-speed monitoring. The control system adjusts the compressor inlet
guide vanes and turbine speed sequentially and ultimately modulates the hot gas bypass valve to maintain
the required leaving chilled water temperature. With
all the conditions displayed-evaporator (suction) and
compressor (discharge) pressures, low suction or high
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ASHRAE Journal - August 2019

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