The Drawbacks of High-Density Storage

A VNA strategy can increase storage space and save capital, but it can also create bottlenecks.

As we all know, land is a finite resource. Particularly scarce is developable land that increases the supply of real property in the marketplace and keeps the price of real estate from increasing.

Because we can't just create more useful land, real property costs — commercial, industrial or residential — continue their steady upward march.

Warehouses and distribution centers, in particular, take up a lot of space and need lots of surrounding land to support inbound and outbound tractor trailers. Anything that can be done to reduce the size of a distribution center reduces its cost as well as the fixed overhead of the company using it.

Since space is so valuable for its rarity, those planning distribution center layouts often consider very-narrow-aisle (VNA) storage systems. VNA storage can increase the storage density of a facility dramatically without sacrificing selectivity. But, if not applied with some wisdom and upfront analysis, a VNA storage system can also become an unexpected bottleneck, constraining a distribution operation from achieving the throughput customers require.

There are several factors managers must keep in mind when evaluating a VNA storage strategy. Some preparation and knowledge can help ensure a facility benefits from maximum density while avoiding potential throughput limitations.

How Narrow?

Most VNA systems have aisles that range from six-feet to five-feet, six-inches. There are narrower systems, such as those applying shuttle forks where a standard pallet (40 × 48 inches) is carried down the aisle with its 48-inch dimension perpendicular to the direction of travel instead of the usual parallel orientation, but let's focus on the more common VNA applications.

Common VNA layouts use turret or swing reach lift trucks to service the aisles. The trucks are so named because the forks of the vehicle turn 90 degrees to the left or right, allowing a pallet to be placed in a rack storage position without the truck turning.

A fixed rail system or floor-embedded wire guidance system allows the trucks to move rapidly and safely in the aisle without hitting the racks. The truck moves only straight forward and backward in a VNA, which is at least three feet narrower than the narrowest reach truck aisle.

The density increase is clear; three feet times the length of a storage aisle (let's say 300 feet) times the number of aisles in the warehouse (let's say 20) is a minimum footprint reduction of 18,000 square feet. At $50 per square foot to construct, that's a $900,000 capital savings through building footprint reduction. If the VNA system is in a facility with a taller clear height (Large turret trucks can service systems in a facility with nearly 60-foot clear heights.), it is possible to reduce the square footage even more. Those savings are significant, even after allowing for the higher price of turret lift trucks, which can cost three times as much as conventional counterbalance or reach trucks.

The Downside

But those savings come with sacrifices. The density increase requires a solid understanding of storage throughput requirements, coordination of workload planning for pallets moving in and out of the VNA system and an ability to ensure that volumes can be spread across the aisles.

While dense, the very nature of a VNA system does not allow more product to be moved in and out than VNA turret trucks can support. While every operation must ensure it has the proper amount of lift vehicles, a VNA operation must ensure the storage configuration has enough aisles to accommodate the combined inbound and outbound peak-volume requirements. VNAs only allow for one turret truck per aisle, while conventional layouts allow for multiple trucks to work in the same aisles. If the combined inbound and outbound storage volumes at peak times demand more trucks than there are aisles in the VNA layout, then the operation encounters an unacceptable bottleneck to fulfilling operational, and most importantly, customer demands.

To maximize the effectiveness of VNA lift trucks, a pickup and delivery (P&D) system is often used. A P&D configuration generally consists of extending every other row of single-deep rack an additional bay or two at the end of each aisle. This allows conventional lift equipment to drop inbound pallets in one of the P&D locations as well as take outbound pallets from the P&D positions after the turret truck has deposited them there. This allows the turret or swing reach to do what it does best — move up and down a narrow aisle at high speed and put away and retrieve pallets in the VNA system.

Turret vehicles, especially larger ones servicing systems between 40 and 60 feet tall, do not move rapidly when they are not being guided inside the narrow aisle. Minimizing aisle changes will increase the throughput capability of the turret lift truck, and the P&D system allows for that.

The P&D system, however, requires solid work-load planning to balance the feeding and alleviating processes at the P&D stations. If there are not enough conventional trucks feeding the P&D stations, the turret trucks will be starved or need to change aisles too frequently. On the other hand, if there are too many conventional trucks feeding the P&D stations (or not enough active turret trucks), the P&D stations will fill up, and the turret trucks will not have enough positions in the P&D stations to output pallets effectively. At the same time, the conventional trucks will not have enough positions to drop off inbound pallets.

Also, if fast-moving product is stored in one VNA, the demand on the turret truck in that aisle may far outstrip the number of pallets that can be retrieved or replenished in the required timeframe. All these situations create constraints that drastically limit the volume that can be moved in and out of storage.

Of course, some operations may not lend themselves readily to VNA applications. Extreme peaks in inbound or outbound volume stress the throughput abilities of a VNA. One solution to this problem is to convert an entire aisle into a P&D station. For outbound peaks, this approach starts with a turret truck at the end of one aisle, from which pallets should be pulled. The truck pulls pallets from storage positions and drops them directly onto the floor in the aisle. The warehouse management system (WMS) sequences pallet pulls down the aisle from the starting end. Then, the turret truck pulls the pallets from their racked positions on either side of the aisle and drops them directly into the aisle, filling the aisle as the truck works its way backward down the aisle and away from the extraction end. A low-level transport vehicle, such as a double walkie/rider, then pulls two pallets at a time out of the aisle and transports them to outbound staging, an outbound processing area or an induction area for a conveyor or sortation system.

The reverse process applies to inbound product, as long as enough rack positions are available to facilitate a sequential putaway as the turret truck moves down the aisle. This dramatically increases throughput in a VNA application, but only in one direction at a time. And, it requires semi-directed putaway logic from a control system directing pallets to an aisle. But, the turret operator can choose the final putaway positions for the pallets. This approach is often called “fill and flush” because it uses each storage aisle as a massive P&D location. Before this method can be used, however, an operation must ensure the requisite WMS functionality is available to support it.

Balancing Demands

While there are significant density advantages inherent in VNA storage systems, proper application depends on finding a delicate balance between operational storage demands and VNA system limitations. Attention to throughput requirements, balancing volume across aisles, planning staffing assignments for feeding and alleviating P&D positions and evaluating WMS support capabilities will ensure an operation gains the density advantages of a VNA storage system without the throughput constraints. Density has drawbacks, but when VNA strategy is properly executed, long-term benefits far outweigh the costs.

Bryan Jensen is a vice president and principal with St. Onge Co. in York, Pa., a material handling engineering and consulting firm. He can be reached at 717-840-8181 or [email protected].

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