Cool valves, piping improve engine reliability when called to burn oil

Presentations and discussion at meetings of the gas-turbine users groups, such as the 7F, often call into view problems owner/operators are having with the liquid-fuel systems on their dual-fuel engines—in particular, unreliable starts and unreliable fuel transfers from gas to oil. A couple of years ago, the sure solution for some users rarely called to operate on oil was to disable or remove their “unnecessary” liquid-fuel systems. A benefit of this approach was less-complex and less-costly annual inspections.

But yesterday is not today in the continually evolving business of electric generation. Many owner/operators, now unable to operate profitably on energy sales alone, must provide the grid ancillary services to boost revenue. Some of these services require dual-fuel capability to assure reliable power production on oil should gas supplies become tight, or unavailable—as happened in New England a couple of winters ago. Thus some users are re-commissioning their liquid-fuel systems; others are installing liquid-fuel capability on gas-only engines, and liquid-fuel systems are a headline item—once again.

Information shared at user-group meetings points to two ways to improve the reliability of dual-fuel gas turbines (GT) on oil starts and fuel transfers: Continuous circulation of oil through liquid-fuel-system components, and active cooling—including the use of water-cooled valves, manufactured by JASC (Tempe, Ariz), to prevent coking of oil in critical check valves and assure proper seating.

The coking problem many users experience with standard liquid-fuel check valves occurs after switching from oil to gas. Oil remaining in check valves, which are located close to the combustion section of the unit, is exposed to high temperatures. Above about 250F that oil is oxidized. The resulting coke coats check-valve internal surfaces (and fuel lines as well) and restricts the movement of valve parts and the flow of oil.

Once this occurs, a check valve will not open and close properly until it is overhauled, which requires special equipment and skilled technicians to assure its better-than ANSI Class 6 seal in the reverse-flow direction. The most common trip during fuel transfer is on high exhaust-spread temperature—caused almost exclusively by check valves “hung-up” on coked fuel.

To get a first-hand look at the issues faced by owner/operators with dual-fuel GTs, the editors recently spoke with a 7FA user who shared experience from a station equipped with five simple-cycle 7FAs and one 2 × 1 7FA-powered combined cycle. All engines at the facility are equipped for dual-fuel firing and have DLN2.6 combustion systems. Plus, all have JASC water-cooled liquid-fuel check valves—14 per GT (one per combustor). JASC valves were installed on the first engine at this site in 2006. Worldwide, more than 500 industrial gas turbines worldwide now are equipped with JASC water-cooled liquid-fuel valves.

The peakers each start 120 to 150 times a year and operate up to about 3500 hours annually these days. The combined-cycle is a mid-range unit. With plenty of gas available, unit run time on distillate typically is less than 50 hours a year—most of that time to keep the liquid-fuel systems exercised and to identify maintenance issues.

Management expected long runs on oil when the polar vortex hit in 2015. Plant personnel prepared for that event with comprehensive fuel-system testing and maintenance, but the threat to gas supply never materialized and the plant only ran a day or two on oil.

The few check-valve problems experienced over the years have been cooling-water related. Water for valve cooling comes from the closed cooling-water system, which recirculates a mixture of water and glycol. The fin-fan cooler for the peakers supplies water at about 130F in summer; that for the combined cycle, about 150F. Such hot water for cooling is not problematic because the goal is to keep the check valves under 250F.

The user interviewed recalled that fuel-transfer reliability was in the low 60s (percent) with standard check valves and the reason why the plant switched to JASC. Today, transfer reliability is in the upper 80s. Note that not all fuel-transfer failures are related to the check valves. Bypass, stop, and control valves, flow dividers, and servos share the blame; plant personnel have to pay close attention to all fuel-system components.

In fact, performance of the water-cooled liquid-fuel check valves have been the least of the plant’s worries, the editors were told. Air in the liquid fuel system was said to be the biggest impediment to reliable operation—it can extinguish the flame in a can and trip the unit. Here’s how air gets into the system: When hot oil cools its volume decreases creating a void. Air entering the system at any point in the fuel-oil system gets pulled into the vacuum pockets.

A patient transfer from gas to oil prevents flame blow-out by air. Gas is always available at this utility-owned plant and operation on oil begins with a startup on gas. While still in Mode 1, oil is introduced and the two fuels are co-fired for about 60 seconds. Then oil is shut off and the unit runs on gas for another minute or so. The first step then is repeated four more times: The fuels are co-fired, and then gas alone, and so on. Next, the gas is shut off with oil running alone, absent the threat of an air pocket.

Installation of the check valves and the supporting cooling-water circuit was not difficult. Plant personnel prefabricated all the water lines after the trial installation, which took about two days per unit. Staff also did the valve installation—typically a day for each engine. Early on, the check-valve cooling circuit was operated with a delta P that was too high: 50 psi. Overcooling allowed unwanted wax to come out of solution. Reducing the differential pressure to 12 psi eliminated the issue.

To assure that the plant maintains fuel-transfer reliability at a high level, check valves are removed at each combustor and hot-gas-path inspection and returned to JASC for servicing—this to avoid a failed fuel transfer caused by a check-valve problem.

After speaking with plant personnel, the editors met with Schuyler McElrath at the JASC booth during the 7F vendor fair to learn more about air pockets and how to avoid them. McElrath is a 25-year GE veteran who specialized in fuel-system design and testing for most of his OEM career.

He said users often think that exhaust temperature spreads are the result of air in the system. In his experience, when a significant amount of air is trapped in the fuel system, the turbine will trip and send a diagnostic alarm stating there was an excessive fuel-flow fault. This trip occurs when the flow divider spins faster than expected because air is being compressed.

McElrath believes that for users experiencing high spreads and/or exhaust-temperature trips during fuel transfers from gas to liquid without excessive fuel-flow faults, the problem likely is caused by fuel becoming increasingly viscous as it transitions to solid coke. He recommends that turbine owners who have upgraded to water-cooled valves run on liquid fuel at least monthly to mitigate this problem.

For new upgrades to water-cooled valves, McElrath suggests relocating fuel piping away from hot turbine casings and using JASC’s recently introduced thermal clamps to provide continuous cooling of fuel lines (Fig 1). They transfer heat from the fuel piping to the cooling-water circuit serving the check valves. Thermal clamps can increase the intervals required for exercising the system from one month to as many as four.

Jasc 1The thermal relief valve (Fig 2) is another new JASC product for liquid fuel systems. It is designed to prevent over-pressurization of the liquid-fuel system when oil is trapped between the stop and check valves and tries to expand when exposed to casing heat.

JASC has been listening to users. To eliminate test firings on oil, thereby saving an ongoing expense as well as reducing emissions, the company has been talking up its Zero Emissions Equipment (ZEE) Performance Test system now in the final design stages.

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