DDG-52AF Vacuum Collection, Holding And Transfer (VCHT) System
DDG-52AF Vacuum Collection, Holding And Transfer (VCHT) System Problems Crew Brief Ernest Durelli, NSWCCD Code 631 30 January 2014 Background DDG-52 VCHT Systems
Two independent VCHT systems per ship (forward and after systems) Sewage collection piping system under vacuum (12-18 inches mercury (Hg)) Vacuum subsystem collects wastes from vacuum water closets and urinals equipped with vacuum-interface-valves (VIVs) Vacuum generated by sewage powered ejectors provided with pumps that are required to be available 24/7/365 VCHT system sewage holding tank at atmospheric pressure Gravity plumbing waste drains (sinks, showers, deck drains, laundry, etc.) above the waterline can be collected in plumbing waste collection tank (normal configuration), VCHT system sewage holding tank, or diverted overboard (in transit or outside restricted waters) Gravity plumbing waste drains below the waterline must be collected in in plumbing waste collection tank (normal configuration) or VCHT system sewage holding tank at all times Ships laundry drains to after VCHT system pump room. These drains should be aligned for collection in the after plumbing waste collection tank. 2 30 January 2014
Background Sewage Powered Ejector and Ejector Pump The sewage powered ejector system generates vacuum by pumping sewage from the bottom of the sewage holding tank by ejector pumps through ejectors, back into the top of the sewage holding tank. The vacuum collection piping system is connected to the suction side each ejector via a suction distribution manifold and a non-return valve at the each ejector suction inlet. Sewage flows through the an ejector nozzle within the ejector body creating a differential pressure which allows the ejector to remove air from the collection piping (the holding tank is not under vacuum). When the ejector system is not running, the non-return valve isolates the vacuum collection piping from the holding tank, which is at atmospheric pressure. The ejector pumps that are used to power the ejectors are controlled by three vacuum switches that maintain system vacuum within operating requirements. 3 30 January 2014 Background Sewage Ejector Pump The Herborner VCHT system ejector pumps are used on CVN-77, LCS-1, DDG-52AF, PC-1, and LPD-17 Classes. This pump experiences heavy service (frequent start/stopping)
under normal conditions, long run times with system problems (vacuum leaks, vacuum pressure switch problems), clogged ejector inlet piping, and are required to be available 24/7/365. Sewage Ejector Pumps 4 30 January 2014 VCHT System Issues Technical Vacuum leaks (piping connections, clogged ejectors)
Vacuum pressure switch issues Excessive foam generation and overboard discharge of foam Excessive ejector pump run times and high ejector pump failure rate (due to vacuum leaks, vacuum pressure switch issues, excessive foam) Gray water system backflooding and overboard discharge of foam from VCHT tank overboard overflow Scale build-up in collection piping (clogged ejector non-return valves, poor drainage) Cost Four ejector pump/motor assemblies per ship New ejector pump and motor assembly cost approximately $60K each Maintenance and repair labor (chemical cleaning, replacing pumps, looking for vacuum leaks, troubleshooting, etc.) VCHT System Availability
Ejector pumps needed for vacuum collection of wastes from vacuum water closets and urinal equipped with vacuum interface valves (VIVs) Ejector pumps required to be available at all times (24/7/365) Safety and Health Potential exposure to sewage (when replacing/repairing ejector pumps, cleaning components, etc.) 5 30 January 2014 Vacuum Leaks and Ejector Clogging System vacuum leaks (vacuum water closet and urinal VIV connections, ejector non-return valve) and ejector clogging
Vacuum leaks are a costly problem associated with VCHT systems since they are often a root cause of numerous material issues and in many instances difficult to locate. System vacuum leaks can cause ejector pumps to run excessively (leading to increased wear and tear) and also lead to excessive foam generation in the VCHT sewage holding tank. One common type of system vacuum leak occurs at the ejectors non-return valve. The non-return valve is the VCHT systems "interface" between the vacuum side of the system (collection piping) and ambient pressure side of the system (the sewage holding tank which on DDG-52 AF ships is NOT under vacuum). When an ejector pump IS NOT running, the non-return valve isolates the vacuum collection piping from the sewage holding tank, which is at atmospheric pressure. Vacuum in the collection piping pulls the non-return flapper up against the seat of the housing. Any buildup of solids or any obstruction can cause vacuum leakage across the valve. The non-return valve has to be removed from the ejector assembly in order to be cleaned. 6 Ejector Assembly Parts Diagram Ejectors 30 January 2014
Vacuum Pressure Switch Issues Vacuum pressure switch issues (clogged and/or incorrectly set) Vacuum switches control the operation of the ejector pump operation and are set at 12 mercury (Hg) (standby pump cut-in), 14 Hg (duty pump cut-in), and 18 Hg (pump(s) cut-out). Direct exposure of the vacuum pressure switch ports to sewage can account for erratic ejector pump operation and excessive runtimes. Erratic ejector pump operation typically involves clogged or malfunctioning cut-in switches and can be time consuming to troubleshoot, while a clogged or malfunctioning cut-out switch typically results in the ejector pumps failing to de-energize. This excessive pump runtime results in increased foam production in the sewage holding tanks and decreased vacuum generation efficiency, due to accelerated wear of ejector nozzles and ejector pump internal components. Severely Eroded Ejector Pump Impeller Vacuum Pressure Switches 7 30 January 2014
Foam Generation Causes and Impacts Foam is generated from the agitation of any detergents/cleaners in the VCHT sewage holding tank contents by the ejectors. Excessive foam is the result of ejector pumps running excessively (because of system vacuum leaks, clogged ejectors, vacuum pressure switch issues, airbound conditions) and/or excessive amounts/high concentrations of detergents/cleaners in the VCHT sewage holding tank. Ejector pumps running excessively also elevates the temperature of the VCHT sewage holding tank contents which also contributes to excessive foam generation. Excessive foam can: be ingested by sewage transfer and ejector pumps, resulting in them becoming airbound. pressurize the VCHT sewage holding tank and the tanks overboard/vent piping. This can
result in backflooding of gravity plumbing waste drains if aligned for collection in the sewage holding tank. discharge overboard or out of tank vent piping. Any discharge of sewage (or sewage contaminated foam) in port is unacceptable (safety and environmental compliance) 8 30 January 2014 Excessive Foam Generation Impact Ejector Pump System vacuum leaks and vacuum pressure switch issues can cause ejector pumps to run excessively (leading to increased wear and tear) and also lead to excessive foam generation in the VCHT tank. Foam is produced from the agitation of any detergents/cleaners in the sewage holding tank contents by the ejectors. This foam can also become ingested by the sewage transfer and ejector pumps, resulting in them becoming airbound (thus unable to move fluid through the ejector nozzles) and any resulting cavitation can damage internal pump components. Cavitation Pitting on Ejector Pump Impeller 9
30 January 2014 Excessive Foam Generation Impact Overboard discharge and out of tank vent piping Foam out of tank overboard on DDG-51 Class ship Typical tank vent termination on weather deck on DDG-51 Class ship (bellmouth fitting and mesh screen) 10 30 January 2014 Excessive Foam Generation Recommended Corrective Actions and Preventative Measures
Proper use of approved fixture (vacuum water closet and urinal) cleaners in accordance with the Ships Hazardous Material List (SHML). Each ship is assigned an applicable type-SHML (or TSHML) providing a listing of hazardous material authorized for use on that specific ship. Proper disposal of waste water generated from routine maintenance evolutions such the cleaning of berthing space and sanitary space decks. This waste water (which can contain high concentration of cleaners) should be disposed of via the ships gray water system and not allowed to enter the VCHT system via vacuum water closets and urinal fixtures. Activation of the VCHT system sewage holding tank system and sewage transfer pumps to lower the fluid and foam levels in the sewage holding tank. The cold firemain water will help knockdown the foam. Note: The sewage transfer pumps may have already become airbound as a result of ingesting foam and may require re-priming.
Locate and fix system vacuum leaks through manual system isolation or use of vacuum leak detector. Ensure the main plumbing waste diverter valve in the VCHT pump room is aligned to collect plumbing waste in the plumbing waste collection tank NOT the VCHT system sewage holding tank. Wastes from plumbing waste sources contain high concentrations of soapy wastes (shampoos, body washes, etc.). Note: The ship's laundry should drain to the after plumbing waste collection tank. Ensure the VCHT sewage holding tank vent termination on the weather deck is not clogged. Excessive foam can also travel up the vent piping and clog the vents bellmouth fitting and mesh screen if the tank overboard scupper valve is gagged closed. 11 30 January 2014 Locating Vacuum Leaks - Manual Method 1) Close the two VCHT system collection piping isolation valves (Figure 1) connected to the suction distribution manifold in the sewage pump room. This will isolate collection system piping and components from sewage pump room piping and components. 12 30 January 2014
Locating Vacuum Leaks - Manual Method (Continued) 2) Observe the VCHT system vacuum gauge (Figure 2); if the vacuum level remains stable, vacuum leak(s) exist in the collection system (piping, valves, water closets, urinals, and VIVs). If the vacuum level decreases, vacuum leak(s) exist in the pump room (piping, valves, ejector non-return valves, etc.). 13 30 January 2014 Locating Vacuum Leaks - Manual Method (Continued) 3) To locate vacuum leaks in the collection system, secure all sanitary spaces and then open one of the two VCHT system collection piping isolation valves connected to the suction distribution manifold in the sewage pump room. 4) Starting at the highest sanitary space and working down, close sanitary space collection piping isolation valves associated with the open VCHT system collection piping isolation valve in the sewage pump room until VCHT system vacuum levels stabilize. Use the ships SDOSS as a guidance for applicable valve locations. The leaks(s) will be located in collection piping or components upstream of the last isolation valve closed.
Vacuum leaks produce sounds in check valves (metallic sound) that are located in collection piping and failed outlet piping (hissing sound) from vacuum water closets. 14 30 January 2014 Locating Vacuum Leaks - Manual Method 5) If no leaks are found, repeat steps 3 and 4 for the remaining VCHT collection system isolation valve connected to the suction distribution manifold in the sewage pump room. 6) To locate vacuum leaks in the sewage pump room, ensure the two VCHT system collection piping isolation valves connected to the suction distribution manifold are still closed. Then, systematically close pump room piping isolation valves to the suction distribution manifold until VCHT system vacuum levels stabilize. The leaks(s) will be located in pump room piping or components upstream of the last isolation valve closed. 15 30 January 2014 Locating Vacuum Leaks
- Vacuum Leak Detector NSWCCD Code 631 accomplished successful land-based and shipboard testing (on some DDG-51 Class ships) of a commercially-available ultrasonic leak detector (presently available in the stock system). This commercial vacuum leak detection device has shown to provide the capability to quickly locate vacuum leaks that cannot be detected by conventional labor-intensive and time consuming methods. NSWCCD Code 631 issued an In-Service Engineering (ISE) Advisory No. 001-10 (VCHT Leak Detection Device (071539ZJAN10)) on this ultrasonic leak detector that provides ordering, NSN, APL, and PMS information for those ships able to procure them. This advisory only recommended the use of the leak detector as a troubleshooting tool and did not authorize their purchase or supply them to the Fleet. Unscheduled Maintenance Requirement Cards (MRCs) were added to affected ship class
MIPs (DDG-51, LPD-17, and PC-1 Classes, and hulls LCS-1, LCS-2 and CVN-77) and SPMIG numbers were generated, to complete a suite of ILS changes (APLs, NSTM 593, etc.) to document the device as originally identified in the advisory and to provide ILS support to those hulls that were able to procure them. 16 30 January 2014 In-Service Engineering (ISE) Advisory No. 001-10 (VCHT Leak Detection Device (071539ZJAN10)) 17 30 January 2014 Vacuum Leak Detector 18 30 January 2014 Current Efforts by NSWCCD Code 631
(Wastewater ISEA) VCHT Instrumentation Isolation Testing (VCHT-IIT) NSWCCD is currently testing non-diaphragm type isolators to determine if they can be used to prevent sewage debris from effecting instrumentation performance, and will test both transducers and vacuum switches to determine compatibility. Testing will result in configuration design proposals compatible with existing DDG-52AF VCHT systems for future validation testing. Shipboard testing would be necessary to develop an SCD that properly address vacuum switch issues, and would contribute meaningful VCHT system data. NSWCCD Efforts Related to Current Advances in VCHT Systems In response to future designs incorporating pressure transducers, NSWCCD Code 631 will test the proposed DDG-113AF pressure transmitter (also on CVN-77) as part of VCHT-IIT. NSWCCD developed a conceptual design to interface pressure transducers with existing DDG-52AF ejector pump controllers, and is building a prototype as part of VCHT-IIT. If both transducers and vacuum switches are compatible with isolation, side-by-side testing can be accomplished during shipboard testing.
A unit that includes pressure transducers could also be used to perform advanced system troubleshooting and validation testing of future proposed design changes (new ejector). 19 30 January 2014
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