Dancing Deaerator

June 1, 2012
The boiler was tripping offline because of insufficient water in the steam drum. Auxiliary-equipment instrumentation and distributed-control-system sensing were inadequate. Earlier investigation and reports indicated the incidents occurred two or three times a month, late in the second shift or during the third shift.

A few years back, I was the energy/utilities manager for a U.S. Department of Defense subcontractor producing structural frame assemblies for military aircraft. Meetings on top of meetings were held every day to track the fabrication, assembly, inspection, and testing of thousands of parts and component assemblies. Any deviation from established shipping schedules (set several months in advance) caused ripples throughout the chain of command (both our company executives and the on-site military-program managers).

The site included five mechanical service rooms. In the newest, a 20,000-lb-per-hour watertube boiler that supplied steam to the anodize-process building mysteriously was tripping offline during back-shift operation, resulting in process delays and a production-schedule fiasco. Steam from another boiler (located about 1,000 ft away in another building) was available, but that line normally was not pressurized.

Problem Analysis
The boiler was tripping offline because of insufficient water in the steam drum. Auxiliary-equipment instrumentation and distributed-control-system sensing were inadequate. Earlier investigation and reports indicated the incidents occurred two or three times a month, late in the second shift or during the third shift.

Six roving boiler attendants worked on a rotating schedule, performing routine duties and safety checks required to meet ASME and state boiler-code regulations. Obtaining reliable information from the boiler attendants proved difficult. Moreover, their direct supervisor was not an experienced boiler man. I was faced with conflicting reports, disinterest, and a lack of accountability. Log-sheet reports would indicate: "The boiler tripped on low water. Took two hours to resolve the low-water issue and get the boiler restarted and back up to 110-psig steam pressure."

The deaerator operated effectively, and there was a duplex boiler feedwater pump set with "auto" startup of the standby pump. The entire boiler room was relatively new (only 5 years old) and fairly well-instrumented and appeared to perform reliably under normal conditions.

I developed an incident-report form and asked the boiler operators to begin more thorough observations and make notes for me to review daily. Several days went by with nothing abnormal.

Upon my arrival at work one morning, the maintenance manager called to report the boiler had tripped over night. I went directly to the boiler room. The unit was back online operating normally. However, I noticed a water leak off to the side of the boiler, near the deaerator platform. It was a threaded fitting on the 1½-in. makeup-water valve. Then, I discovered the "free-standing" deaerator structure had moved about 2 in. from the normal position. The deaerator tank held about 900 gal. at normal water level. The complete fabricated assembly (with tank, pumps, controls, and piping) weighed more than 8,000 lb. How could the unit have moved?

Shift change had occurred; the third-shift operator had gone home, and the first-shift operator was in one of the other mechanical rooms performing routine duties. The chemical-process employees were busy with production-delay and rework issues.

That night, I visited the plant during the third shift to discuss the incident with the boiler attendant. He said he had responded to a low-steam-pressure alarm and found the boiler tripped. The feedwater pumps also were tripped, and the deaerator water level was low. He had opened the manual bypass valve around the zeolite-softener module to get water into the deaerator. He said the makeup-water pressure seemed low and that it took quite a while to get the deaerator water level back to normal.

Later, the first-shift operator told me the third-shift operator had neglected to close the manual bypass valve after water level was restored. The deaerator filled with water, and a violent water-hammer rumbling condition shook the tank, moving the structure 2 in. Suddenly, it became apparent to me why the Air Force general in charge of the military-contract programs had demanded that an energy/utility manager be hired.

More-thorough analysis revealed:

  • The boiler safety controls were responding to a system malfunction.
  • The deaerator-tank controls seemed to be working properly.
  • The softener units indicated a pressure differential of only 10 psig.
  • The feedwater pumps were operating effectively.
  • The makeup-water control valve was functioning properly.

After another day of review and critique, I revisited the water-supply-pressure issue. It had been disregarded because water pressure in the boiler room (to the zeolite softeners) normally was 80 to 90 psig. Twelve-inch and 10-in. pipelines connected to a 36-in. city water main that supplied the entire facility. The street pressure was a constant 100 to 110 psig. Could an anomaly have been occurring within our plant water-distribution system?

I ordered a portable water-pressure recorder (with a seven-day chart) and installed it on the city water line in the boiler room. Four days later, we noticed the pressure had dropped from 80 psig to 20 psig for over an hour during the third shift. Review of production operations in the adjacent chemical-cleaning building revealed one of the chemical tanks had been drained and flushed during the night. The production-team leader said replenishment of certain chemical tanks was scheduled as needed. Replenishment normally was scheduled during the back shift because of chemical-concentration permit regulations concerning effluent to the municipal sanitary treatment plant. Recent production flow through the anodize process had increased, and tank cleaning had become more frequent.

We monitored the next scheduled draining, flushing, and refilling of a 10,000-gal. chemical tank. Water pressure in the entire chemical building dropped from 90 psig to 20 psig for more than two hours during flushing and refilling. Time was crucial for this procedure so that the anodize-process schedule could be maintained in the other chemical tanks.

The 2-in. city water line in the boiler building was connected to a main 6-in. line that supplied the chemical building. It, too, had incurred a pressure drop to 20 psig.

We finally identified the cause and result of the problem. For the boiler system:

  • Zeolite-softener pressure drop normally was 8 to 10 psig.
  • The makeup-water control valve had a differential of 6 to 8 psig.
  • The deaerator water spray valves had a 5- to 7-psig differential.
  • The deaerator (mounted 10 ft above grade) operated at 5 psig.

A city water-supply pressure of at least 35 psig was necessary to get water into the deaerator.

A findings review with operations management confirmed my suspicion: Modifying the chemical-tank cleaning procedure was not an option; we had to solve the boiler-shutdown issue without a process-system modification.

It took three months to design, quote, receive, and install a duplex constant-pressure booster-pump system for the 2-in. city water line in the boiler building. In the meantime, the boiler attendants and chemical-operations supervisors worked closely to mitigate the low water-supply-pressure conditions.

The skid-mounted, self-contained system had on/auto/off operational mode. The 7.5-hp booster-pump unit continuously monitored water pressure, automatically started, and maintained 85 to 90 psig to the softener units when city water-header pressure dropped. After one hour with a supply-header pressure above 50 psig, it would shut down and remain in auto-standby mode.

In complex manufacturing facilities, utility-equipment performance sometimes can be compromised by uncontrolled operation of specific process systems.

If management responsibility for production processes and supporting utilities is split between staff organizations, identifying and correcting operational anomalies can be challenging. Turf issues may arise.

Gather facts, investigate conflicting reports, review design and operational data, develop a list of probable causes, and eliminate obvious non-issues. Then, it is up to you to develop a practical solution management will accept.

Gary Wamsley, PE, CEM
JoGar Energy Services
Alpharetta, Ga