MM&D MAGAZINE, SEPTEMBER/OCTOBER 2011 PRINT EDITION: Simplicity is a virtue in designing a green facility, and using high-density storage systems is a tested method to reduce DC size.
In the design of pallet storage systems, there’s a conflict between product access and the need to maximize capacity. Most systems achieve success by eliminating storage aisles. In a conventional single-pallet-deep selective racking system, storage aisles account for half of the usable space. By eliminating aisles, usable storage capacity is increased and building size reduced.
Items with the largest product per SKU in terms of pallets are the best for high-density storage. A key design issue is the amount and type of order picking. If only full-pallet order selection or handling is required, there’s limited need for designated order-picking slots. Common high density storage systems include: Bulkpile or block storage; double-deep racking; drive-in or drive-through racking; pushback racking; and flow racking.
The number of standard unit loads that can be stored vertically varies among systems, since two of the systems—pushback and flow racking—use a sloped rail to take the pallet to the front of the rack face.
Double-deep pallet racks also require more clearance.
Unlike other high-density systems that use racking for vertical storage, block or bulkpile systems depend on the pallets’ stacking capacity and the unit load contents. This limits the application to ceiling heights of 20ft or less.
Differences in the way high-density storage systems operate affect the capacity and selectivity of each system. In three systems—double-deep, pushback and flow racking—the lift truck remains at the access aisle. For double-deep racking, a specialized truck with an extendible fork puts the pallet into the second storage position, but the storage system is limited to a maximum of two pallets deep. With block storage and drive-in racking the lift truck must make a right angle turn to the storage aisle and enter the storage lane or tunnel to retrieve stored pallets.
Mixing it up
All high-density storage systems risk mixing different types of pallets (different SKUs or lot or date codes) required within the same storage channel. For drive-in and bulkpile storage, the storage channel is the entire vertical tower that a lane or tunnel forms. For the other three systems, selectivity is increased by the number of vertical levels because each level functions as a separate storage channel.
With bulk pile or block storage, pallets or unit loads are stored on top of each other without the use of support racking. Unlike most storage systems this one has few fixed elements except for building columns and aisle lighting.
In terms of cost-per-pallet, double-deep storage is the cheapest (rack cost only) form of high density racking system because it uses standard racking. It’s limited to a maximum doubling of storage density over regular racking and requires specialized deep reach forktruck equipment. Like the majority of high-density storage systems, double-deep racking is a last-in-first-out (LIFO) based storage system.
The next most expensive high-density storage system is drive-in or drive-through racking. It’s twice as expensive as double -deep racking and can be stored many pallets deep. In terms of total cost—including the building—it’s competitive with double-deep storage.
Drive-in racking can be used with standard lift trucks and is similar to bulk pile storage. But it doesn’t have the same stacking height limitation. Pushback and flow racking pallets are automatically indexed to the pick face once a pallet is removed. This indexing speeds up pallet putaway and removal particularly at upper rack levels.
One SKU at a time
Both drive-in and bulk pile racking are limited to one SKU per storage lane or tower. Pushback and flow racking can have a different SKU for each level. The greater number of SKUs to be stored in a drive-in system means there are a greater number of empty slots compared to a pushback system. This results in 10 to 20 percent lower occupancy than with a pushback or flow rack system.
With flowrack or pushback, a slope is needed to provide the indexing that conveys the pallet to the rack face. This can decrease usable vertical heights particularly in deeper systems of five or more pallets.
Compared to other high-density storage options pushback racking provides the following considerations. It’s twice as expensive on a per-pallet position basis and limited to six pallets deep. Compared to flow racking it’s half the price on a per-pallet position basis. Pushback racking is also capable of multiple system design options in the vertical plane.
Compared to flow racking it’s less expensive, even compared to the newer bearingless systems. But in an order-picking application, picking and restocking happen in the same aisle, compared to flow racking where picking and restocking take place in different aisles. While flow racking is the most expensive, it’s the only system that provides first in first out (FIFO) storage.
Dave Luton (firstname.lastname@example.org) is a consultant in the greater Toronto area.