Coal, Gas, Hydroelectric

Fiberglass Pipe Replaces Wood Stave Penstock

Issue 7 and Volume 112.

Jackman Hydro Station, built in 1926, is one of nine hydroelectric plants owned by Public Service of New Hampshire (PSNH). The facility is fed by the Franklin Pierce Reservoir, which was created by the hydro facility’s dam and surrounding earthen dikes. The water elevation in the reservoir is managed according to spring run-off.

Until recently, a large wooden penstock served as the main water supply for the hydro facility, which serves the Hillsborough, N.J., area. Prior to the recent improvements, 5,000 feet of wood stave penstock extended from the Franklin Pierce reservoir to a surge tank upstream of the powerhouse. This 7.5-foot diameter penstock was made up of vintage wood stave sections, circa 1926. The penstock was repaired in 1954 and in the 1970s, yet the downstream sections still leaked badly. This leakage caused icing problems during winter months and required constant maintenance. In 2003, the penstock had a significant rupture that impacted abutting properties. To prevent future failures and ensure reliable operations of the hydro facility, more repairs were necessary. In a series of contracts, PSNH decided to replace the remaining wood stave penstock with a new pipeline.

Kleinschmidt, headquartered in Pittsfield, Maine, specializes in energy and water resource projects and was retained to develop a feasibility study and engineer a replacement pipeline. Kleinschmidt initially evaluated five pipe material options for the penstock replacement: wood stave, concrete, high-density polyethylene (HDPE) plastic, steel and fiber reinforced polymer (FRP). Factors that led to the final determination included the structural reliability, corrosion resistance, hydraulic capacity, ease of installation (pipe weight) and the pipe material’s performance history. Based on Kleinschmidt’s evaluation and recommendations, PSNH chose the fiberglass pipe option and purchased centrifugally cast fiberglass reinforced polymer mortar (CCFRPM) pipe manufactured by HOBAS Pipe USA, Houston.

HOBAS field service crew assisted the installation contractor. Click here to enlarge image

“The corrosion resistance and relative stiffness of FRP pipe meant it could be supported on the existing grade and half buried, rather than placed on saddles or completely covered in a buried trench.” said Keith Martin, a project engineer and civil/structural engineer with Kleinschmidt. “This design flexibility, combined with a longer projected service life lead to a competitive total construction cost for the FRP alternative.”

The lightweight fiberglass pipe sections were carried to the point of installation and assembled using a single excavator. Click here to enlarge image

The lower frictional resistance of CCFRPM compared to other materials allowed the replacement pipe’s diameter to be reduced to seven feet without additional head loss, Martin said.

The pipe was half buried in the shallow trench of the existing penstock, requiring minimal excavation costs and little environmental disturbance. Approximately 300 feet of the new penstock were supported above ground on steel saddles to accommodate the topography, as well as minimize the impact to wetlands that the penstock crossed.

HOBAS CCFRPM is manufactured using a computer controlled centrifugal casting process. The higher axial strength necessary for the aboveground installation is achieved by adding reinforcement in the longitudinal direction during the manufacturing process.

CCB Inc. of Westbrook, Maine, installed the pipe. The job site’s remote location posed some pipe installation challenges. In one phase of the job, 1,020 feet of 84-inch diameter pipe was unloaded near the roadside. The individual joints were then carried to the point of installation with an excavator. “We had no truck access to the installation point,” said Newell Porter, CCB project manager. “The first part of the installation was through a curve where 10-foot joint sections were connected and then deflected to make up the curve,” Porter said.

The coupling joint used on this project was a HOBAS FWC pressure joint. It is commonly used in direct-bury applications and also for above ground installations such as penstocks. It is a structural filament-wound sleeve over wrapped and mechanically locked to an internal full-faced elastomeric membrane. The sealing design includes both lip and compression elements so the joint is suitable for both non-pressure and pressure service up to 250 psi. “In our case, we air tested each joint after assembly with a 10 psi air test. None of the joints leaked. Once the line was in operation, we were required to visually inspect each and every joint for a sign of leakage and there was none,” said Porter.

With the help of subcontractor JML Trucking and Excavating of Errol, N.H., CCB completed the project two weeks ahead of schedule. “At the end of the project, it was found to be approximately seven times cheaper per foot to utilize the fiberglass pipe option partially backfilling the penstock than to support it above ground,” said Martin. Hydraulic advantages and longevity of the line will also provide lifelong savings.

Cutting Silica Analysis Costs and Maintenance

Silica analyzers in modern power plants alert operators to harmful silica concentrations in the water-steam turbine cycle. In the boiler, silica forms silicate deposits that interfere with heat transfer and are difficult to remove. In the turbine, silica builds up on the blades and causes drastic decreases in efficiency. Power plants with high pressure boilers closely monitor silica concentrations to avoid these problems.

Plants monitor silica with online analyzers, grab-sample testing or some combination of the two. Both options are expensive. Online analyzers have high capital and maintenance costs. The cost of the reagents alone can exceed $2,000 per year per analyzer. Grab sample analysis of just one batch of samples can take a half hour or more of valuable operator time.

In an attempt to curtail these costs, engineers at PPL Generation’s newest energy center in Lower Mount Bethel, Pa., decided to try a silica analyzer recently introduced by ABB Instrumentation. This compact analyzer, the Navigator 600, can monitor up to six streams simultaneously.

A cost justification prepared for the plant estimated that the Navigator 600 analyzer would save nearly $28,000 annually by replacing three existing silica analyzers and eliminating grab sample testing. At this rate, the payback time for the new analyzer would be eight months.

Combined Cycle Generation

The Lower Mount Bethel plant, completed in 2004, is a 600 MW nominal natural-gas-fired combined cycle mid-merit peaking facility. It sits on the west bank of the Delaware River about 80 miles north of Philadelphia. Two gas-fired combustion turbine generators, two heat-recovery steam generators (HRSGs) with duct burners and one steam turbine generator in a combined-cycle configuration provide efficiencies ranging up to 60 percent.

Heat from the two gas turbines’ exhaust gas is used to generate steam in each of the HRSGs. The boilers operate at three different pressures: 2235, 415 and 75 psi (high pressure, intermediate pressure and low pressure).

The greater the pressure, the greater the chance of silica carryover from the boiler water into the steam. As the curve in Figure 1 indicates, at 1,800 psi, a silica content of 1,000 parts per billion (ppb) in the boiler water will produce carryover of 20 ppb in the generated steam.

Silica carryover from the drum to the steam increases with increasing drum pressure. Graph courtesy of PPL Generation LLC Click here to enlarge image

According to Bernard H. Herre, PE, PPL Generation’s chemical laboratory manager, silica carryover is not an issue in the low and intermediate pressure boilers. “But carryover increases sharply with higher pressures,” he said. “At 2,000 psi, about 5 percent of the silica in the boiler water carries over into the steam.” So operators at the Lower Mount Bethel plant aim to keep the silica content in the high pressure boiler water below 200 ppb, preferably below 100 ppb. This assures that the steam silica concentration will be less than 10 ppb, a safe level for the turbine.

Herre said that most of the silica in steam comes from carryover but that sometimes mechanical entrainment of boiler water into the steam, such as would happen during high water levels or frothing, can add to the problem.

“Silica contamination of the water/steam cycle can occur in several other ways,” said Herre. He cited the following possibilities:

  • Dirt and debris get into components when they are opened for maintenance.
  • Raw water leaks from the condenser and introduces silica into the cycle.
  • No matter how pure, all makeup water contains some dissolved silica. Resins in the demineralizer eventually become exhausted, raising silica content in the makeup water even more.
  • Microscopic undissolved silica particles may pass through filters and the demineralizer and get into the boiler. These particles dissolve in the high pressure boilers, producing a sudden jump in measured silica concentration.

Herre said that operators deal with high silica content in various ways, depending on conditions. The best way, he said, is to avoid putting the silica into the boiler in the first place. For this reason, only ultrapure water is fed to the boiler. To reduce levels of impurities that do get into the boiler, operators periodically discharge (blow down) boiler water to waste. As a last resort operators can reduce the boiler pressure until the silica level is reduced by blowdown.

Monitoring Silica Content

Monitoring silica at the plant was accomplished originally with three analyzers—one on each high pressure steam line and one on the condenser discharge. In addition, grab sampling of boiler water blowdown from the boiler drums and the makeup water amounted to 32 manual silica tests per day.

During the trial, the ABB Navigator 600 silica analyzer was temporarily mounted on a hand truck within a trailer. The reagents sit below the analyzer while the standard and cleaning solutions are inside it. Click here to enlarge image

Over a four-month period, the new Navigator 600, temporarily mounted, proved that it was accurate, consumed low amounts of reagent and required little maintenance. It is estimated that the annual reagent cost per analyzer will be half that of the plant’s existing analyzers.

Maintenance labor for changing reagents and recalibrating (typically a one to two hour process) should decrease significantly. Chris Westlake, ABB Instrumentation’s analytical industry manager, said that the new unit periodically cleans and calibrates itself, requiring virtually no maintenance except replacement of the reagents every three months. “Reduced handling of reagents lessens the chances of contamination and contact with hazardous chemicals,” said Westlake.

Many plants consider silica analyzers to be maintenance intensive, requiring tubing, o-rings and fittings to be replaced monthly. Westlake claims that the Navigator 600 will require only five to 10 minutes of annual maintenance to service its peristaltic pump.

In the permanent installation, the Navigator 600’s six streams will monitor high pressure steam and boiler water from each of the two HRSGs as well as condensate from the condenser and makeup water from the condensate storage tank. “This will eliminate the need for any additional analyzers and for grab samples,” said Westlake. The six output signals, will run to the plant’s data historian computer.

Next Steps

The Lower Mount Bethel plant plans to add a second Navigator 600 silica analyzer for the demineralizers that produce the makeup water. The demineralizers for water treatment sit in a portable trailer. Each train of demineralizers consist of three tanks: cation resin, anion resin and a final “polisher,” which is a mixed-bed of both resin types. The demineralizer processes filtered water from the river to provide the makeup water. Acid and caustic regenerates the resin beads as necessary. The plant’s silica specification for the demineralized ultrapure water is 10 ppb.

The new two-stream Navigator 600 analyzer will monitor silica in the anion and mixed-bed tanks of the demineralizer train that is in service. It will provide a timely indication that the resins are becoming exhausted and must be replaced. At that time the plant will order a new demineralizer trailer containing fresh resins.

Click here to enlarge image