Coal, O&M

Converting Flash Steam to Megawatts

Issue 8 and Volume 109.

In March 2004, a large oil-fired power plant in the Northeastern United States had a predicament; flash steam released from a rooftop vent was inappropriately being viewed as environmental pollution by neighboring citizens. To compound the situation, the station’s fuel oil heating system was not providing sufficient heat for optimal delivery of No. 6 fuel oil to the boilers. Management contemplated adding a fuel oil booster heater to improve plant performance, but the additional flash steam resulting from the modification would increase the already objectionable plume from the condensate recovery tank’s vent. Management also considered a new condensate waste heat recovery system that would capture energy from the hot condensate and utilize it as both a source of energy to improve plant performance and a means to reduce the steam plume, mitigating the environmental concerns.

During the following months, the plant’s project manager worked closely with SEA Consulting Services/Altran Corp. engineers to design a system that maintained the hot condensate from the new (and existing) fuel oil boosters at sufficient pressure to avoid generating flash steam until after its thermal energy could be transferred to the feedwater supplying the fuel oil steam evaporator (FOSE).

“In the original design, steam was generated in the fuel oil steam evaporator and distributed through the fuel oil heating system to the fuel oil tank farm, fuel line heat tracing and fuel oil heaters,” said Robert Hammelmann, SAE Consulting Services/Altran Corp. senior project manager. As heat was transferred to the oil, the hot condensate was trapped and returned to a common condensate recovery tank, maintained at near-atmospheric conditions through a six-inch diameter vent.

“Because hot condensate is essentially a saturated liquid, any reduction in pressure caused a portion of the flow to convert to flash steam,” Hammelman added. “As the condensate passed through the heater drain discharge trap and headed toward the recovery tank, pressure reduction and flashing began.”

The liquid portion of the condensate collected in the tank, while the flash steam portion was released to the atmosphere through the vent. The remaining liquid condensate was used as the source of feedwater supply to the FOSE. The condensate storage system replenished the water lost through the flash steam.

Modification Development

“We redesigned the fuel oil condensate return system to extract the thermal energy from the hot condensate before it reached atmospheric conditions in the condensate recovery tank,” said Steve Genca, SEA Consulting Services/Altran Corp.’s chief mechanical engineer. “We did this by using a shell-and-tube heat exchanger that simultaneously cools the hot condensate as it heats the feedwater supply to the FOSE.” This heat exchanger was designated as the condensate cooler/feedwater heater (CC/FH).

To provide satisfactory thermal and hydraulic performance, the system pressure must be maintained above saturation pressure to prevent the hot condensate from flashing to steam prior to reaching the heat exchanger. “We had to determine the hydraulic resistance in the piping system and use that information when selecting steam traps and a backpressure-regulating valve for the condensate line downstream of the heat exchanger,” Genca added.

Using the fuel oil booster heater design as an example, the design team selected the steam trap based on minimizing flashing and providing modulating flow control through the trap. They also selected a pilot-operated thermostatic trap. The booster heater engineering data indicated 10,965 lb/hr of 100-psig steam input, therefore 10,965 lb/hr of condensate would be generated. At this flow rate, the following pressure drops would occur:

• 1.4 psid (psi differential) through the booster heater (tube-side), per manufacturer’s data

• 2.0 psid through the trap, per manufacturer’s data

• 4.5 psid through the piping system, calculated

• 0.2 psid through the CC/FH (tube side), per manufacturer’s data

In theory, the initial steam pressure (100 psig) less the sum of the losses listed above (8.1 psid) is the maximum backpressure (91.9 psig) required to obtain the desired flow rate of hot condensate through the piping system. A backpressure-regulating valve was installed to produce this backpressure. The remaining 91.9 psi pressure drop occurs through the valve and piping to the condensate recovery tank.

“Setting the backpressure-regulating valve to maintain 91.9 psig only worked if the steam pressure in the header remained at 100 psig,” said Hammelmann. “Any reduction in steam header pressure resulted in less than the required differential pressure needed to move the mass flow of condensate through the system. Therefore, we installed two pressure transmitters and a modulating positioner on the valve.”

Click here to enlarge image

As Figure 1 illustrates, the pressure transmitters are located in the steam supply piping just prior to the fuel oil booster heater and in the condensate line just upstream of the backpressure-regulating valve. The pressure transmitter output signals are fed to a controller that sends a 4-20 mA signal to the backpressure-regulating valve positioner. The control point was initially set at 8.1 psi pressure differential between these two points in the system, but the setpoint can vary from 5 psid to 15 psid to allow for minor variances in the pressure drop versus flow data calculated by the engineer and equipment manufacturers.

Condensate processing at the six main fuel oil heaters was designed in a similar manner. The existing steam traps were used, but the discharge orifices were resized to pass the required flow (3,750 lb/hr each) at the reduced pressure differential. In total, 33,465 lb/hr of hot condensate is available as a source of energy at the CC/FH.

Energy Recovery

The predicted energy recovery in the CC/FH is shown in Figure 2. Energy transfer is based on 30,000 lb/hr of condensate/feedwater flow.

Click here to enlarge image

In this instance, the increased energy input to the feedwater provides a commensurate reduction in the energy input requirements for the FOSE. The result is greater net steam flow through the turbo-generator system due to reduced extraction steam that is used as the source of energy input to the FOSE.

Flash Steam Venting/Make-Up Water

“Uncontrolled steam venting results in both real and perceived issues associated with the plume. Because the plant’s condensate storage system replaces the condensate inventory lost with flash steam, any reduction in flash steam venting decreases the condensate make-up required. This is an important factor, especially for stations that rely upon municipal water as a supply source,” said Genca

The amount of flash steam generated from hot condensate as it is depressurized depends upon the temperature and pressure conditions before and after discharge. As the saturated liquid temperature is reduced, the excess sensible heat is absorbed in the water and transformed into steam as determined by its latent heat of evaporation. This can be quantified using steam tables and the following equation:

Percent flash steam loss = [(hf1 – hf 2) 2] x 100 (where hf is the enthalpy value listed in the steam table)

Hot condensate is normally near saturated water conditions. For example, if the hot condensate is initially 100 psig at 320 F and it flashes to steam under atmospheric conditions in the condensate storage tank at 0 psig at 212 F, the percentage of inventory lost to flash steam is:

(290.53 – 180.07) / 970.3 = 11.4 percent

If the pressure and temperature of the hot condensate can be reduced to 91.9 psig at 260 F, the percentage of inventory loss to flash steam then becomes:

(228.95 – 180.07) / 970.3 = 5.0 percent

This represents a 56.1 percent reduction in flash steam loss, which, based on 30,000 lb/hr condensate flow, equates to 16,830 lb/hr less steam plume off the vent, and 50,523 gallons per day less condensate demand.

Operational Results

“Once the new design features were installed, the desired results and improvement to the fuel delivery system were achieved, and the condensate waste heat recovery system began performing at its rated conditions,” said Hammelman. “The system also mitigated the perceived environmental concern.”