"Ensuring Optimal Operation of Condensate and Feed Water Systems in Power Plants"

Steam Boiler – Condensate and feed water system – Operating Checks & Troubleshooting .

In this blog ,Learn how to maintain optimal performance in power plant condensate and feed water systems. Discover essential operating checks, troubleshooting tips, and best practices to prevent costly downtime and ensure efficient system operation.

If you're responsible for keeping a condensate and feed water system running smoothly, you must be thoroughly familiar with normal operating conditions for the system so you can distinguish between usual and unusual conditions.

Making regular checks of levels, temperatures, pressures, flows, and valve positions throughout the system as it operates helps you detect problems early so that major trouble and costly downtime can be prevented.

Specific concerns associated with the operation of your facility's condensate and feed water system are usually covered in your plant's standard operating procedures and in the operating manuals for the individual pieces of equipment.

In this part, however, we'll look at some basic operating checks that are common for most condensate and feed water systems.

A typical condensate and feed water system includes several multi-stage centrifugal pumps. Checking the pressures, temperatures, and lubrication for the pumps during normal operation of the system is a routine operator responsibility.

Checking pump suction and discharge pressures periodically helps ensure that each pump is operating correctly and that proper flow through the system is maintained. Proper bearing lubrication is critical to keep the pumps in a condensate and feed water system in good working order.

To make sure that a pump's bearings are being properly lubricated, you should check for overheating, excessive vibration, contaminated lubricant, and low lubricant level. One way to check for overheating or excessive vibration is by touching the bearing housing. You can also use an instrument called a pyrometer to measure the temperature at the bearing housing and a vibration meter to measure the amount of vibration.

To check the flow, level, and condition of the lubricant, sight glasses are commonly used. If lubricating oil in a sight glass has an unusual color, this could be a sign that the oil has been contaminated and should be replaced.

For this pump, a gauge measures the temperature of the lubricating oil leaving the pump's bearings. If the reading isn't within the normal temperature range, there's generally a problem that should be reported.

Many pumps use cooling water to cool the pump's bearings. When you check these pumps, make sure that the valves in the cooling water lines are open.

A centrifugal pump often uses seal water to help keep fluid from leaking out of the pump. During normal operation, you should check the seal water to be sure it's working properly.

Also, if funnel drains are used for seal water leak off, check the drains for proper seal water flow.

If the flow is greater than normal, this could indicate a leaking seal.

If a pump has a minimum flow requirement, a flow rate less than the minimum causes higher turbulence and greater friction, which generates heat in the pump.

So, recirculation lines and valves are used to maintain minimum flow and prevent overheating. If flow through the pump goes below the minimum flow requirement, the valves in the recirculation line are opened to maintain minimum flow.

But when flow through the pump is above the minimum level, the valves in the recirculation line should be closed.

Condensate pumps usually have suction and discharge vent lines that are connected to the main condenser. In most cases, the valve in the discharge vent line is closed after the condensate pump has been started and is operating normally.

A low hot well level occurs when the amount of condensate leaving the main condenser is greater than the amount of steam that's condensing.

This condition could cause the condensate pumps to lose their prime.

A high hot well level occurs when the amount of steam condensed in the main condenser is greater than the amount of condensate removed by the condensate pumps. This condition could flood the lower sections of the tubes in the main condenser and reduce the condenser's overall condensing ability.

In an extreme case, water from the main condenser could get up into the turbine and destroy its moving blades.

A level control loop is used to maintain the correct level of condensate in the hot well. Basically, condensate flow out of the hot well is decreased if the level falls below a preset minimum and increased if the level exceeds a preset maximum.

A typical condensate and feed water system has several shell and tube heat exchangers or closed heaters. Some may be used as condensers, but most are used as low intermediate or high pressure heaters for the condensate and feed water.

The levels, temperatures, and pressures for all the closed heaters should be checked periodically during normal system operation to make sure that these operating variables remain within predetermined normal ranges.

We'll look at some operating checks for a high pressure closed feed water heater.

Most of the checks for this type of heater also apply to low and intermediate pressure condensate or feed water heaters. To monitor the level in the shell side of the heater, you can usually check a sight glass on the heater.

An abnormal level in the sight glass could indicate a problem that should be reported.

Temperature gauges indicate the temperature of the feed water leaving the heater and the temperature of the extraction steam entering the heater. For proper system performance and the overall efficiency of the plant, it's very important that these temperatures stay within their normal operating ranges.

Pressure gauges are used to monitor the pressure of the extraction steam that enters the heater and the feed water that passes through the heater. The reading for the extraction steam pressure normally varies with turbine load. The greater the turbine load, the higher the extraction steam pressure.

Feed water heaters and many condensate and feed water systems are installed in trains or strings that are monitored as a unit during normal operation of the system. In this arrangement, the final feed water outlet temperature is commonly used as one way to monitor the performance of the heaters. The final feed water outlet temperature is the temperature of the feed water coming out of the highest pressure feed water heater on its way to the boilers economizer. Checking this temperature is the quickest way to get an overall picture of how well the feed water heaters are doing their job. If the final feed water outlet temperature is at the expected level for the given load, then each of the feed water heaters in the train is probably transferring heat at the proper rate. But sometimes a heater within the train may be transferring heat at an improper rate, even though the final feed water outlet temperature is at the expected level. So periodically you should also check the outlet temperature of each heater in the train.

The deaerator, which is an open heater, and the hot surge or deaerator storage tank that's associated with the deaerator must also be monitored when a condensate and feed water system is in operation.

Check the level of water in the hot surge tank. There's usually a level sight glass on the tank that's used for this purpose. If the level is too high, water could flow into the deaerator and in extreme cases even into the extraction system piping. If the level is too low, the net positive suction head for the pumps downstream of the tank is reduced. As a result, the pumps could be damaged. Monitoring the performance of the deaerator is important for maintaining overall system and plant efficiency.

Check the appropriate gauges for the proper extraction steam temperature and pressure. Look for any signs of leakage and make sure that all the valves for the deaerator are in the proper positions.

Watch for excess venting from the deaerator. That can cause a major loss of heat energy and fluid from the deaerator, which will decrease plant efficiency. Also watch for insufficient venting. That will allow oxygen and other gases to remain in the condensate and feed water system and cause problems downstream.

Problems can occur with any of the components in a condensate and feed water system, including the pumps, heaters, storage tanks, and their auxiliaries, such as valves, vents, motors, couplings, and piping.

In this part, we'll look at typical examples of dealing with some operating problems that are common in most condensate and feed water systems. But keep in mind that you'll need to review your plant's standard operating procedures to learn the exact steps to follow in your facility.

Many operating problems in a condensate and feed water system are relatively simple to correct. However, a problem in one component usually affects the operation of other components in the system as well. Consequently, some operating problems can require considerable troubleshooting skills. Several heaters are included in a condensate and feed water system,

so heater problems are a typical operating concern. We'll look at some problems in a closed high-pressure feed water heater. Improper venting of the heater can cause air binding.

When this occurs, air is trapped in either the tube side or the shell side of the heater. Air that collects in the water boxes on the tube side of the heater can restrict the flow of water through some of the tubes. The restricted flow causes the velocity of the feed water to increase because the same amount of water must go through fewer tubes. The increased feed water velocity can cause the terminal temperature difference, or increase, although the drain cooler approach, or DCA, remains normal.

Another indication of air binding in the water boxes is an increase in the differential pressure, or delta P, across the heater. If air binding is indicated by either an increased TTD or an increased delta P, venting the air from the water boxes should return the feed water flow and pressure to normal.

Air binding also occurs if air and gases collect on the shell side of the heater. The increasing volume of air blankets the tubes and prevents steam from contacting them. This in turn reduces the amount of heat transferred by the heater. As a result, the feed water outlet temperature decreases. The TTD however, increases because the extraction steam temperature remains the same even though the feed water outlet temperature has gone down. The efficiency of the drain cooler isn't affected, so the DCA remains normal.

You can usually correct the problem of air binding on the shell side of a heater by venting the shell side to remove the air. The TTD should then return to normal. As a result of the restricted heat transfer caused by a high heater level, the TTD increases. The DCA, however, usually remains normal because the flow of drips through the drain cooler may not change or it may be less than normal.

To determine whether an increased TTD is caused by air binding or by a high level, check the sight glass for the shell side of the heater. If the level in the sight glass is high, then the problem is most likely caused by an abnormal level, not by air binding.

 If a heater's level is too low, steam is able to blow through the heater and out the drain. In turn, the flow of extraction steam increases and this can cause erosion inside the heater and eventually tube failure. Because the low level allows steam to flow through the heater's drain cooler without condensing, a low level causes the DCA to increase drastically while the TTD typically remains at or near normal.

Tube leaks are another common operating problem associated with closed heaters like this high-pressure feed water heater. Even small leaks can significantly increase the volume of water in the drips, causing the lower tubes in the heater to be blanketed with feed water. Tube leaks also make the heater's level control valve open farther than normal. As a tube leak becomes larger, the volume of water flowing into the shell side of the heater can exceed the level control valve's capacity.

Then the emergency drain valve must be used to control the level in the heater. A large tube leak is a serious problem that calls for immediate attention because it can allow water to enter the turbine. To prevent damaging or destroying the turbine, you should isolate and bypass the heater as soon as possible. Of course, operating with even a relatively small tube leak is inefficient, so it's important to find small leaks early and get them corrected. Small tube leaks may be difficult to detect though because they don't show up on instrumentation, but some small leaks may become noticeable when you calculate the TTD and the DCA for a heater.

In many cases, operators must determine exactly which heater in a train is not operating properly. In this example, one heater in a train of high-pressure feed water heaters is malfunctioning. As a result, there's an increase in the temperature difference, or delta T, across the higher pressure heaters downstream from the malfunctioning one.The increased delta T causes more heat to be transferred in the downstream heaters, making up some of the difference caused by the heater that's not operating properly. To locate the heater that's not operating properly, you should generally start by checking the final feed water outlet temperature to see if that value is within the normal range.

Then work backwards, noting the heater outlet temperature for each heater in the train. If the difference between the actual reading and the expected one becomes greater with each heater, there's a malfunctioning heater upstream. To pinpoint the exact heater that's causing the problem, follow a step-by-step troubleshooting procedure, which includes checking the temperature rise across each heater, calculating their TTDs and DCAs, and checking the positions of their normal drain valves. Then comparing the actual conditions with the expected normal conditions for the heaters at the given load.

A typical condensate and feed water system also has several pumps, so operating problems often involve troubleshooting pump malfunctions. For example, if notices that although this boiler feed pump is operating at top speed, the flow of feed water to the boiler is inadequate.

To keep the situation from getting worse, a second boiler feed pump is brought online. This increases the feed water flow to an adequate rate to keep the boiler operating properly. Then company procedures are followed to analyze and correct the problem. Detailed troubleshooting could determine that the inadequate feed water flow was caused by an operating problem in the first boiler feed pump.

On the other hand, further analysis could show that the pump is working properly and a malfunction elsewhere in the condensate and feed water system could be the cause of the inadequate feed water flow.

 

"Thanks for Reading!”