Avoid Assembly Hassles

Skid-mounted systems can minimize construction headaches and expedite projects.

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Figure 3: Efficient Deployment Required


Hutchison-Hayes makes skid-mounted centrifuges for deployment in hostile environments, such as off-shore drilling rigs. This mandates an efficient modular design to ease integration.


One key disadvantage of off-site construction is the need for coordination among different vendors to ensure commonality of components, methods of construction, and control systems. While it is possible to mandate that all skid builders follow strict and unwavering standards, in practice this can increase costs and lengthen schedules.

According to NNE's Larsen, up-front planning with all skid builders is essential to ensure that necessary standards are met while allowing each skid builder to maintain internal efficiency. On a recent project, NNE specified DeltaV controllers from Emerson Process Management, Austin, Texas, for all skids, but allowed the skid builders to use components of their own choosing in other areas.

NNE found that trade-offs must be made when standards collide with costs and schedule.

One throat to choke

Some call it single-source responsibility, others refer to it as having one throat to choke. In any case, clients always know where to turn if a skid doesn't work as designed. This is usually not the case when a company decides to construct a system on-site, unless the construction is awarded to a firm on a complete, engineer-procure-construct design-build basis.

"Skid-mounted systems provide the customer with a single source of responsibility that includes all mechanical and electrical components for the system," says Shawn Harris, engineering manager for Denver-based Metron. Metron provides packaged pumping systems to municipal water districts, power plants, and city park departments. "Most customers prefer packaged systems to eliminate many of the integration problems that arise from purchasing components from multiple vendors and attempting to stick-build at the job site," adds Harris.



Metron officials say one potential customer elected to take the stick-build approach and later regretted the resulting headaches and finger-pointing among the suppliers.

When choosing a supplier for its control and electrical systems, Metron also wanted a single point of responsibility from its supplier. They eventually chose Schneider Electric, Reuil-Malmaison, France, because of the firm's broad range of integrated offerings.

Plug-and-Play Installation

A skid-mounted system typically arrives at the job site fully tested and ready for operation. On-site installation work is minimized, often to an extreme degree.

Enterprise Automation is a control system integration firm based in Bridgewater, N.J., that works extensively with skid builders such as Pacific Consolidated Industries (PCI), Santa Ana, Calif. PCI builds skids that produce industrial gases.

"PCI's nitrogen skid is completely self-contained and does not even require utility hookups," says Dave Panish, president of Enterprise Automation. "It generates its own power, electricity, compressed air, and instrument air. Just hook up a high-pressure discharge hose for nitrogen output, perform a three-step start sequence, and you're done. Load demand is simply determined from the client's nitrogen back-pressure and flow."

Other PCI skids require somewhat more in the way of on-site connections, but the principle is the same. "Skid-mounted systems can be installed and commissioned in much less time than a similar system built on-site," adds Panish. "This allows the customer to be up and running much faster and with less of an impact on its facility and production schedule."

Most skid-mounted systems require interface connections to existing plant control and instrumentation systems. Hard-wired connections demand precise definition of all upstream and downstream analog and discrete interfaces. Any changes to these hard-wired interfaces can result in significant modifications to standard skid-mounted system interfaces.

Another problem with hard-wired interfaces is the sheer number of wires that often must be run from a skid to an existing plant control system. Digital interfaces alleviate these problems by allowing large amounts of data to be transferred over a single communications cable. These digital interfaces are very flexible and can easily accommodate changes in interface connections.

PCI skid-mounted systems use PLC-based controls from AutomationDirect, which allow for many types of integration with existing plant control systems, from simple digital I/O handshakes to more complex communication drivers.

Hutchison-Hayes' centrifuge systems for oil rigs must be plug-and-play so as not to interfere with existing operations. "The complete system needs only two power drops," reports Hensley. "Communications to our Square D control system can be as simple as discrete alarm contacts and local paging. However, the control panels can be equipped with a complete Ethernet-based embedded web server that provides remote control and troubleshooting capabilities."

Skid-mounted systems speed commissioning

One of the main attractions of skid-mounted systems is expedited control system commissioning and start-up. The very nature of modular construction forces skid-builders to perform many commissioning and start-up tasks prior to delivery.

There is a definite alignment of interests between skid builders and their clients with off-site skid testing. It is much cheaper for a skid builder to perform testing off-site and to minimize on-site commissioning and start-up efforts. This dovetails with client desires to have delivered systems up and running as soon as possible.



Despite this convergence of interests, clients know some issues cannot be addressed until a system is running at full production in their facility. However, skid builders' familiarity with their own processes and control systems enables quick and efficient commissioning and starting up of systems. My old employer could completely check out a multi-million dollar system in the shop in less than a week. On-site start up also took less than a week.

Regulated Complications

Commissioning and start-up can be especially problematic in regulated industries where control system validation is required. In such cases, it can be particularly beneficial to purchase skid-mounted systems with control systems that can be quickly validated on-site.

Separations specialist Millipore Corp., Billerica, Mass., (Fig. 4) uses common control platform (CCP) software to control large-scale automated systems, says Tom Dennen, technology manager of automated separation systems. "CCP software has been audited by an independent third party and complies with FDA guidelines for 21 CFR Part 11."

Figure 4: Drop-In Validation


Millipore offers membrane filtration and chromatography systems for purification and separation of pharmaceuticals. The company uses compliant control software and fully tests every system prior to delivery. Each system includes a complete documentation and validation support package.


In addition to providing compliant control software, Millipore fully tests every system prior to delivery. Each system includes a complete documentation and validation support package including Installation Qualification/Operation Qualification test protocols. All documentation complies with guidelines published by the GAMP Forum, a joint interest group promoting the use of computer and control systems within healthcare industries, and its JETT consortium of pharmaceutical manufacturers, equipment suppliers and consultants involved with the validation requirements of the pharmaceutical industry.

Modular construction is also becoming an important strategy for bioprocessors, and was critical in starting up Biogen's new full-scale production facility for the monoclonal antibody, Amevive, in Research Triangle Park, N.C. According to Pharmaceutical Manufacturing, a sister publication of Chemical Processing, 20% of the facility's mechanical, piping and electrical work took place offsite, as did construction of process units for cell culture, buffer preparation and hold, clean-in-place and bioreactors. Since all the modular systems could be tested before they were brought online, the strategy saved considerable time and effort.

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