above the feedpump to provide the neces-sary head to drive the pump. Depending onthe size and type of plant, the valve may bereducing an inlet pressure as high as 5500psig to an outlet pressure of about 150psig. These extremely large pressure dropscause very high-energy cavitation that willdestroy a standard control valve trim in avery short time. In some plants, the recir-culation line runs to the condenser, whichcreates an even higher pressure drop for

the valve to handle. As a result the valvehas to handle both cavitating and flashingconditions.One of the main causes of failure of thesevalves is due to particulate in the flow-stream. Many times, this is due to improp-er blow techniques following valve installa-tion that cause weld slag and other debristo become lodged in the inlet passages oftoday’s drilled hole or stacked disk designs(See Figure 2). In many older plants, there

POWER GENERATION

New technology eliminates plugging of

boiler feedpump recirculation valves

By John Wilson, Fisher Controls International, Inc.

The boiler feedpump recirculation valvetypically sees the most severe service

conditions of any control valve in a powerplant. As water passes through the feed-pump, it picks up heat. This temperaturerise can cause cavitation, which is the for-mation and subsequent collapse of vaporbubbles in the liquid flow stream. If thevapor collapse occurs near a solid bound-ary, a pump impeller or the pipe wall forexample, the collapse mechanism can dam-age the boundary surface. Protecting thepump against cavitation is possible by main-taining a minimum flow through the pumpto minimize any temperature rise. This isaccomplished by installing a recirculationsystem around the feedpump.Figure 1 shows a typical feedpump recircu-lation system. The feedpump recirculationvalve takes feedwater from the boiler feed-pump discharge and recirculates it to thedeaerator in conventional drum style boilersor to the low pressure drum in combinedcycle units. The deaerator or low pressuredrum is typically located several stories

Figure 1. Boiler feedpump recirculation system

As the combined cycle power boom continues to thrive, ques-tions regarding performance of existing units continue to sur-face. This is especially true as the discussions regarding theresurgence of coal fired technology spur on. In these plants, theboiler feedpump recirculation valve is one of the main work-horses. If this valve does not perform up to expectations, it cancause severe pump damage, degraded unit performance or evenbring a unit offline.

Figure 2. Plugged drilled hole and stacked disk anti-cavitation trim

OCTOBER 2001 Valve �� World 1www.valve-world.net

POWER GENERATION

also can be issuesregarding ironoxide buildup oreven particulatefrom degradingupstream equip-ment.Issues with plug-ging are a con-cern, but manytimes the partic-ulate can passthrough the inletpassages only tocause severeerosion damageto the throttlingand shutoff sur-faces of thevalve trim.

Damage to the shutoff surfaces leads to de-creased unit performance as flow thatshould be producing steam is routed backthrough the main feedpumps, leading to ex-cess pumping requirements as well as caus-ing severe damage to the valve body. Re-cent experiences have shown that leakingrecirculation valves can cost a plant up toUSD 275,000 each year in lost capacity andmaintenance costs.

The new solutionBecause of the issues noted above, FisherControls developed the Dirty Service Trimor DST (See Figure 3). DST provides cavi-tation control for applications with en-trained particulate that could plug the inletpassages or cause severe erosion damage toconventional anti-cavitation trim. The DSTdesign uses combined axial and radial flow

path that features large openings that allowsflowing particulate up to 2” in diameter.Because of the need for tight shutoff, thisdesign incorporates a protected seating sur-face that separates the shutoff and throttlinglocations. All significant pressure drop istaken downstream of the seating surface. Asa result, the seating surfaces are not wornaway by throttling control action (unlessthrottled near the seating surface for ex-tended lengths of time) resulting in im-proved leakage performance over time.Also, the throttling areas are not requiredto have the superior surface conditions oth-erwise needed for tight shutoff. Figure 4shows a cross–section of DST highlightingthe protected seating surfaces and pressurestaging technique.As seen in Figure 4, the DST staged pres-sure reduction design takes the majority ofthe pressure drop in the initial stages of thetrim, which dramatically reduces the avail-able energy of the fluid leaving the finalstage. This eliminates the possibility ofhigh-energy fluid impingement on the valvebody, and in cases where there is entrainedparticulate, erosion damage to the valvebody and downstream equipment is elimi-nated.The DST technology also subjects all clear-ance flow to a staged pressure drop. Thiseliminates the possibility of fluid going di-rectly from P1 to P2, which is a situationthat can occur within linear cage-style anti-cavitation trim (See Figure 5). In the linear

trim, the highvelocity im-pingement onthe seating sur-faces results inpoor controland loss ofshutoff. An exampleDST applica-tion involvesan older power plant in Michigan. Theplant had to bring the 450 MW unit downapproximately every other month becauseof plugging to the recirculation valve. DSTwas installed in the valve in 1996 and hasyet to be opened because of a lack of ca-pacity or lack of shutoff.DST was also used to retrofit three existingvalves in a New England nuclear powerplant. The plant was attempting to increasethe flow rate through the condensate recir-culation valves, but the existing valvescould not provide the necessary capacity.Because of its large open flow paths, theDST solution provided the required capaci-ty and cavitation protection, while other of-ferings required the installation of largervalves.

Additional problem applicationsBoiler feedpump recirculation is not theonly service requiring cavitation protectionwhile still allowing the passage of large par-ticulate. In a gas processing plant, the

Figure 3. Fisher Dirty Service Trim

Figure 5. Clearance flowcavitation seen in linearanti-cavitation trim

Figure 6. Before cleaning (left) and after cleaning (right) pictures of DST in a water injectionpump recycle applicationFigure 4. Cross-section of DST (Dirty

Service Trim)

2 Valve �� World OCTOBER 2001 www.valve-world.net