Queue Block

The Queue Block is used to create a buffer to hold items during the simulation run. As with other blocks in Process Playground, the Queue Block has the ability to hold and pass every item in a model when "Allow All Items" is enabled, or it can be set to only pass specific, unique items when the item type added to the block.

The defaults for "Allow All Items" does not have "Initial On Hand" property.

As shown in the image above, the Queue Block has relatively few input parameters. They are:

• Max Items: This sets the maximum number of items allowed in the block. Adjust this value if you are modeling a queue with limited capacity.
• Probability FIFO: This value, which can range from 0 to 1, sets the queue’s first-in, first-out (FIFO) discipline. The default value is 1, which means the queue will operate in perfect FIFO order. If you change the value to 0.5, the queue will take the item at the head of the line 50% of the time, acting in FIFO mode, then randomly select an item elsewhere in the line the other half of the time. Setting the value at 0 will create all random, non-FIFO behavior.
• Unbatch All Items: Checking this box will unbatch any batch that arrives at the queue and add items to the queue individually.
• Balk At This Time: This optional parameter allows you to define how long an item should wait in the queue before it “balks” and leaves the queue. When an item balks, it does not leave the queue in the normal path; instead, it is redirected to another queue that the next parameter has identified.
• Balk Based On: This parameter allows you to choose whether to Balk based on the Block Cycle Time (BCT) or Total Lead Time (TLT)
• • BCT: An item will balk if it spends the amount of time in "Balk At This Time" in the block. e.g. If the Balk At This Time Paremeter is C(4) m (a constant of 4 minutes), then an item will leave the Queue Block if it spends 4 minutes in the Block.
  • TLT: An item will balk if the amount of time it has spent in the process up until it arrives in this Queue Block is greater than or equal to the time in Balk At This Time. e.g. If the Balk At This Time Paremeter is C(4) m (a constant of 4 minutes), then if the item has been in the entire process for longer than 4 minutes, the item will leave this block.
• Balk Target: This parameter allows you to select the target queue for balked items. These items will be forced into the target queue, regardless of its quantity limits.
• Set Priority To: This parameter allows you to change the priority of items that flow through the queue when they leave, not when they enter. For more information, see this post on Item Priorities.

When you add a specific item, or items, to a Queue Block, an additional parameter called "Initial On Hand" becomes available.

A specific item on the Queue Block has an "Initial On Hand" property.

Initial On Hand specifies how many items of this specific item type should be created at the start of the simulation run. This option allows you to pre-load your simulation with items (WIP) at the beginning of the run. This parameter is helpful in modeling manufacturing processes where the production line starts with WIP in place, in contrast with an “empty” process (e.g., making a drink at a coffee shop).

Also note that the item you add to the Queue has most of the parameters listed above. This allows you to set unique values based on each type of item in the Queue Block. For example, you can direct one type of item to a Target Queue after balking at 4 minutes wait time, while another may go to a different Queue after balking at 3 minutes wait time.