Francisco Alcala is a member of ISA and an instrumentation specialist for CDM. BSEE from Universidad de Oriente Venezuela and Operation Management Specialist from IESA Venezuela. 18 years of experience in I&C design, integration, and maintenance in the water/wastewater, petrochemical, and beverage industries.
Jacob Moser is a member of ISA and an instrumentation specialist for CDM. He has a BSEET from DeVry University in Columbus Ohio. He has 13 years of experience in I&C design, panel design, PLC programming, SCADA configuration, instrumentation calibration and maintenance in the water/wastewater and manufacturing industries.
Pumping stations also called lift stations , are normally designed to handle raw sewage that is fed from underground gravity pipelines . Sewage is fed into and stored in an underground pit, commonly known as a wet well.
Sanitary sewer overflow (SSO ) is an event where untreated sewage, from a sanitary sewer, is discharged into the environment prior treatment.
Combined sewer overflow (CSO), is an event where a combined sewer system discharges wastewater and stormwater directly into a river, stream, lake or ocean.
Reliability: can be defined as “the ability of an item to perform a required function specified by its intended purpose, under stated conditions during a given period of time".
A similar survey over 46 cities in 2001-2002, developed by the Water environment Foundation publish under “Efficient Redundancy Design Practices WERF Report publish in February of 2003). :
“ Equipment redundancy is practices improve reliability through the provision of standby equipment or processes that reduce the risk of failure in order to meet water quality regulations or guidelines”. i.e. Orange County Utilities (OCU) applies redundancy design to their facilities to meet regulatory requirements and to reduce operational costs by reducing process-unit downtime, their operating costs have in 3 years. “
Efficient Redundancy Design Practices WERF Report publish in February of 2003).
“ Accomplish documentation of realistic approach's to redundancy guidelines that have practical goals having in considerations high-, medium-, and low-cost options, or short- and long-term projects.”
“ Develop a "how-to implement redundancy design practices”
“ Examine the effective use of redundant instrumentation and automation, which can have a significant impact on design and operation. Study how it can be used to reduce capital costs or eliminate plant expansions and reduce operations and maintenance costs”.
Sewage pumping stations are typically designed so that one pump or one set of pumps will handle normal peak flow conditions.
Pumping redundancy is built into the system so that in the event that any one pump is out of service, the remaining pump or pumps will handle the designed flow.
The storage volume of the wet well between the 'pump on' and 'pump off' settings is designed to minimize pump starts and stops, but is not so long a retention time as to allow the sewage in the wet well to go septic.
Due to the environmental impact of SSOs, lift station pumping must be reliable and continuous including the ability to respond to varying flow demands regardless of day, hour, or availability of electricity.
Industrially rated components have been designed and tested to withstand the rigors of an industrial environment. These components can withstand, wide variations in temperature, dirty environments, poor power qualities, high moisture, and high vibrations.
Not Exceeding the Environmental Limits of the Components
Temperature is a major contributor to premature component failure in industrial control panels. Each component will have acceptable temperature ranges for proper operation. The closer you operate at the extremes the more likely a failure will occur.
Moisture can lead to premature component failure. Condensation can cause corrosion or shorting out of components.
Proper Mounting of the Components
Air flow is important. Every industrial component has a manufacturers recommended spacing and orientation for proper air flow around each component. This ensures proper heat dissipation.
Vibration should be considered when selecting how a component should be mounted within an industrial enclosure as well as location of the enclosure. If the components are subjected to excessive vibration, component failure can occur.
Ingress of moisture or corrosive gases will occur if great care is not taken when installing the control panel. Even with the best location and components if there is an excessive amount of moisture or the presence of corrosive gases component failure is inevitable. All gaskets need to be present and checked. Proper conduit entry into the enclosure using the proper type of hubs is imperative.
Transient Voltage and Surge Suppression
Transient voltages and Surges are a major contributing factors to component failure. A lightning strike near the panel or near the electrical feed can destroy the components inside a control panel. Proper TVSS selection and proper grounding and bonding of the enclosure will help protect the components.
Backup system by nature are low demand Application, with a restore rate that is not constant. For failures not detected until a periodic inspection and test, the restore rate is zero until the time of the test. If it is discovered the system is operating successfully, then the probability of failure is set to zero.
“ In practice we define at TUV 2 a test as a diagnostic test if it fulfils the following three criteria:
1. It is carried out automatically (without human interaction) and frequently (related to the process safety time considering the hardware fault tolerance) by the system software and/or hardware;
2. The test is used to find failures that can prevent the safety function from being available.
3. The system automatically acts upon the results of the test”
Diagnosic And Periodic Testing To Improve The Availabilty Of Backup Systems 2. TUV Technischer Überwachungsverein (German: Technical Monitoring Association)
The Probability of Failure on Demand is the probability of a given safety instrumented Function (SIF) cannot perform its safety function when is a need it. (This same concept can be apply to redundant or backup system)
PFD(t) = e − λ DU ⋅t + e − λ DD ⋅MTTR
PFD(t) will be close to “1” on the mission time. PFDavg also would be close to 1.
If is possible to reveal the dangerous undetected failures via testing of the SIF (backup Function). This called “proof test” in the safety standard argot.
Proof tests or diagnostic executed periodically in order to detect DU failures in Time serves to reveal dangerous failures which otherwise remain undetected.
Diagnostic and Periodic Testing on the Reliability of Backup or redundant Systems. 3. The Effect of Diagnostic and Periodic Testing on the Reliability of Safety Systems, W. Velten-Philipp, M.J.M. Houtermans, 1st international safety symposium USA, Cleveland, OH, USA, June 14-15, 2005
Marginal investment of implementing test routines for Redundant or backup system versus cost associate with SSO or CSO Mechanical or power failures as well as miscellaneous events (which account for 16% of all events) may be considered preventable spills due to lack or failure of the redundant or backup components. These events could also be avoided by diagnostic tests followed by corrective actions. Report to Congress, Impacts and Control of CSOs and SSOs. Publisher: EPA 833-R-04-001. Publish Date: August 2004.
Marginal investment of implementing test routines for Redundant or backup system versus cost associate with SSO or CSO The Michigan Department of Environmental Quality (DEQ), Combined Sewer Overflow (CSO) & Sanitary Sewer Overflow (SSO) Annual Reports 2006, 2007 and 2008. Avoidable Fails
Marginal investment of implementing test routines for Redundant or backup system versus cost associate with SSO or CSO
From 2006 to 2008 an average of 19 SSO (4.94%) of preventable spills were found.
Analyzing several emergency response plans issued by different cites and published on the Internet the average cost per event was estimated at $6,000 to $12,000.
Estimated cost of 19* $9,000 = $171,000. Including a Diagnostic and Testing Routine (DTR) as part of the lift station’s design, it may help to reduce this cost the 60% (level of success of DTR) to $102,000.
Marginal investment of implementing test routines for Redundant or backup system versus cost associate with SSO or CSO The average marginal investment of adding a DTR to new or upgraded lift stations was approximately $1,000 per pump station. If this rate is applied to the 359 lift station (Amount of Reported Lift Station of the State of Michigan). The total investment to include DTR will be approximately $359,000. Using the data to calculate the Net Present Value with a discount rate of 4.5% the investment associated with the DTR can be expected to be recovered between two to three approximately.
Marginal cost of implementing test routines for Redundant or backup system versus cost associate with SSO or CSO
The results are theoretical and vary depending on the following factors.
Level of success of DTR.
Diagnostic and test coverage.
Diagnostic and test frequency.
Marginal cost of DTR project.
Type of technology (PLC, SCADA, Telemetry System)
Level of integration and Report.
Number or Lift stations. (Economies of Scale)
Operational costs associated with the DTR Task.
Inclusion of a DTR as parts of the operations and maintenance procedures.
The specification of redundant or backup components is a common practice in the water/wastewater industry.
Following the guidelines set forth by UL, NFPA, and TUV, and implementing an automatic testing regime, a highly available system can be installed with minimal cost impact.
It has been proven that diagnostics and testing will decrease the average probability of fail on demand.
The diagnostic and testing routine can be implemented as a task in a new or upgraded I&C project where the test coverage and frequency may be carefully determined in order maximize the benefits and avoid system upset.