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Boiler Water Feeding -- On/Off vs. Modulating

May 9, 2017

The question facing many consulting engineers today is whether to specify on/off operation or modulating continuous feed when boiler size is between 250 and 1200 horsepower. To answer this question, it is important to understand the difference between the two.

Fig. 1 shows a typical on/off piping arrangement for a single- or multiple-boiler application.  With this piping scheme, a single- or multiple-pump arrangement (one is standby with manual switching) is dedicated to a single boiler, and its operation is triggered by the low-water-cutoff and pump control on the boiler, which is electrically tied into the pump circuit and actuated when a specific level is reached in the boiler. 

Today's Boiler May Fig 1

    Fig. 1 

Compare this piping scheme to one using the modulating continuous feed control (Fig. 2), and feeding based on the water level in the boiler as it gradually reduces due to the evaporation of water to steam leaving the unit. 

Today's boiler May Fig 2

    Fig. 2 

In this case, the pumps are working to a common (pressurized) manifold serving all the boilers connected to it and drawing from a single-feed tank (vented receiver or deaerator) as was the case with the on/off arrangement noted above.  

With the modulating continuous feedwater piping scheme, the pumps are all running constantly at full rating with individual modulating feedwater valves controlling the amount of flow to the boiler based on its evaporation rate. In this case, as the feedwater valve throttles toward closed, the piping includes a bypass (or recirculation line) between the pump and the feed tank to keep the pump from dead heading. 

ADVANTAGES AND DISADVANTAGES

ON/OFF OPERATION

Due to its relative simplicity, it is less expensive to buy and install an on/off piping system compared to a modulating one. Material and labor costs are lower because there is no modulating feed valve, no bypass line and smaller pump horsepower.

However, there are considerable disadvantages with the on/off piping system, beginning with stress on the boiler itself.  

Many of these boilers are fed from vented receivers that can vary greatly in the feedwater temperature based on the amount of hot condensate returning, and/or if the tank includes a pre-heating steam diffuser, which could increase the temperature to 180 – 200 degrees F maximum.  This being the case, the boiler is instantly fed with a large slug of relatively cold water (boiler operating at 100 psig = 338 deg. F).  This causes the metal in the feed connection area to suddenly contract putting inordinate stress on the welded connection.  Cracks will ultimately occur causing leakage and eventually a boiler repair with commensurate downtime. 

A second disadvantage is starting and stopping during many hours of operation that puts considerable stress on the pump motor, shaft and bearings, which leads to costly repairs and possible downtime.  

Lastly, the on/off feedwater operation often causes problems with the boiler’s water level as it demonstrates dramatic swings (sink & swell) and consequential boiler trips on low water or carryover of water into the steam system. The carryover can add to the possibility of water hammer within the steam system, which can be both destructive and dangerous.

MODULATING OPERATION WITH CONTINUOUS FEED

When comparing the on/off with the modulating continuous system, we find the modulating system offsets all the disadvantages of the on/off system. 

With the modulating continuous system, the feeding of the boiler water is closely matched to the steaming rate of the boiler thereby mitigating the stress problem.  This metered feeding of boiler water also provides a more stable water line and fewer, if any, trips and certainly drier steam because carryover is better controlled if not fully eliminated.

The other important advantage with the modulating continuous piping arrangement is the possibility of employing key value-added options to the boiler system to make it more efficient and reliable.

It starts with considering a different low-water cutoff and pump control using magnetostrictive technology (Fig. 3).  This system employs a red brass rod with imbedded wire and titanium float that monitors and displays the float’s movement, sending an analog (4-20 ma) signal to the modulating feedwater valve to move toward open or close relative to the steaming rate of the boiler.

Fig 3

     Fig. 3 

The control also signals the operator in the event of high-level or low-water conditions as well as advises the operator when the column should be blown down and whether it was blown down successfully.

Another key option one might consider is adding variable frequency drive (VFD) to the feed pump motors.  A VFD drive dramatically reduces the electrical draw on the motor because the draw is reduced by the cube of the speed reduction. The VFD is mounted on or near the motor and can be retrofitted or added to new feed systems, mounted and wired as a package.

In addition to the VFD drive, a pressure transducer has to be added between the pump and the feed valve(s).  This transducer will send a signal to the VFD to increase or slow down based on the feed valve(s) opening or closing, thereby increasing or decreasing flow and pressure in the common manifold piping. 

The last value-added option to consider with the modulating continuous system is a PLC-based control system. See Fig. 4.

Fig 4-1 Fig 4-2

     Fig. 4 

With a PLC-based control system, as shown in Fig. 4, all the burner-management and combustion-control systems can be tied into a single platform, including the level control and the VFD drive(s).  Not only does this make the wiring complexity less arduous, but it also enables excellent system control and information to be gathered and trended, including trouble shooting. This saves fuel, electricity, service costs and costly downtime.  

The disadvantage of a modulating continuous system is its cost. 

Using a modulating system requires more piping and controls, which increases capital spend, and includes the bypass line(s) modulating feed water valve(s), and larger horsepower motor(s) because of additional piping and the feedwater valve pressure drops.  Adding the recommended options, including the VFD, Level Master and C-B Hawk control, increases capital costs as well.  However, when looking at these additions from an internal rate of return (IRR) perspective, the savings often offsets the cost in an acceptable period of time.

Steve Connor is an industry expert in steam- and hot water-generation with more than 50 years of experience. He retired from Cleaver-Brooks as Director of Technical and Marketing Services. Steve can be reached at sconnor@cleaverbrooks.com.

Article courtesy of Today's Boiler
Download: Today's Boiler May 2017