Avoid Assembly Hassles

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

Onsite construction is used to build or expand most process plants. Whether the project is done in-house or outsourced, the sequence of work is similar: design, facility construction and modification, and process or system construction and installation.

However, a growing number of companies today are turning to turnkey, automated, skid-mounted systems and modules. The systems may offer a host of advantages, reducing overall costs and project timeframes, when compared to onsite construction.

Keeping, and using, secrets

One key attraction for skid-mounted systems is that they allow operating companies to take advantage of a vendor's proprietary or patented technology. Many "skid builders" have spent an extensive amount of time developing their processes, and only make these innovations available with their skids.

For instance, when I worked for a skid builder from 1986 to 1991, the company had developed a patented software system to perform closed-loop control of sulfur trioxide injected into the exhaust stack of coal-burning power plants. The only way we allowed customers to get this technology was via purchase of skid-mounted units.

Other skid builders have similar stories. "We have over 60 years of experience providing spray systems for virtually every industry. Our understanding of spray applications led us to develop patent-pending control system software specifically for spray applications," says Bill Kohley, the vice president of Wheaton, Ill.-based AutoJet Technologies.

Although many industries use spray systems, these systems are a specialized niche and are generally not a primary plant process. So they are ideal for outsourcing to a skid builder. "Our engineers spend a lot of time in customer plants, and they continually see problems like overspray of expensive coatings, misting, and product rejects. Most customers do not have the background in spray technology to develop effective spray system software," adds Kohley.

Mobile Process Technology (MPT), Memphis, Tenn., has similar reasons for offering its advanced separation technologies on skids. MPT's advanced separation technologies (Fig. 1) provide waste-stream elimination, resource recovery, and remediation to the chemical, petrochemical, polymer, pharmaceutical, and general manufacturing industries.

Figure 1: Delivery & Pick-Up


Mobile Process Technology's proprietary separation technologies provide waste-stream elimination, resource recovery and remediation to many industries. The skid-mounted units often are leased to customers.

Offsite offers benefits, but requires up-front planning

Having a system built offsite provides a number of advantages, particularly in construction safety. The ideal way to reduce the number of OSHA-recordable incidents is to reduce on-site construction hours. Off-site construction in the controlled environment of a skid builder facility should be safer than similar work performed at an operating plant.

Labor efficiency is another benefit of off-site construction. Operating plants introduce a host of inefficiencies including security gate check-in, compliance with union work rules, work-permit procurement, and coordination with plant operations and other crafts. This means that a craftman's 8-hr day at an operating industrial plant typically translates into no more than six hours of productive time at the actual work site.

Not only can off-site craftsmen devote more hours to actual work, they are often more efficient because they work on the same types of systems every day. "Skid-builder personnel are extremely efficient because they are always building similar equipment," says Flemming Larsen, automation project manager for Novo Nordisk Engineering (NNE) in Copenhagen. "Skid builders often use an assembly-line approach to manufacturing, which is much faster and less expensive than on-site custom and one-off construction."

NNE designs and builds pharmaceutical plants, and uses pre-built skids and modules whenever possible (Fig. 2). When skids are used in new construction projects, overall construction time can be greatly reduced. "Buildings and utilities can be constructed while skids are being built instead of waiting until the building is ready for occupancy and then starting construction," adds Larsen. "On a recent project, this trimmed the schedule from 32-36 months to 18 months."

Figure 2: Parallel Processing

Novo Nordisk Engineering designs skids and modules in pharmaceutical plant construction whenever possible. A recent project schedule was trimmed from 36 months to 18 months because module-building was done in parallel with facility building.


Reliance on skid builders also reduces on-site interference among different processes and systems. "When construction takes place on-site, it is often necessary to construct separate systems in a more sequential fashion as compared to off-site skidded systems, which can all be built at the same time," adds Larsen.

When processes must be added to existing plants, skid-mounted systems are often the only good answer. Houston-based Hutchison-Hayes' centrifuge skids (Fig. 3) are often purchased for installation on operating oil-production platforms. "The very nature of this business environment dictates that the equipment have an efficient skid-mounted modular design in order to be integrated into the drilling system with as little effort as possible," observes Gary Hensley, executive vice president of Hutchison-Hayes.


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.

Show Comments
Hide Comments

Join the discussion

We welcome your thoughtful comments.
All comments will display your user name.

Want to participate in the discussion?

Register for free

Log in for complete access.


No one has commented on this page yet.

RSS feed for comments on this page | RSS feed for all comments