Valve Blocks

A valve block can be used to switch flow from one output to another. There must be exactly one input and two outputs.

To configure a valve block, in addition to the common block properties, you will need to describe how the block behaves. The first step is to assign a universal reliability definition (URD) in the Universal Reliability Definition area. The URD uses a model to describe the block's reliability characteristics and may also include a corrective task and/or scheduled tasks.

To assign a URD to a block, you can either select an existing URD or create a new one. Click the URD field and then click the arrow to display the URD wizard. In the wizard, you can click Select Existing URD to display a list of the available URDs. You can also click Create New URD to open the Universal Reliability Definition window; in this window, specify the failure model and any associated tasks, then click OK to create the new URD and apply it to the block.

Once a URD has been assigned to the block, you can view and/or edit its properties from the URD wizard by clicking the View/Edit icon.

You can remove the URD from the block by clicking the Remove icon in the URD wizard.

You can also add, change or edit the components of the URD (i.e., the model, corrective task and/or scheduled tasks) directly from the URD area. Be aware that any changes you make here will apply everywhere that the URD is used.

If no URD is assigned to the block, you can create a new model for the block by clicking the Model field and then clicking the arrow. This opens the Model wizard, which allows you to define a model for the block; this has the effect of automatically creating a new URD that uses the new model and assigning it to the block.

In addition, you will need to specify the following:

  • Operation

    • Set block as failed if selected, indicates that the block is "off" or absent from the system. An X will be displayed on the block to indicate that it is failed. The block will be considered to be failed throughout the entire simulation and no maintenance actions will be performed (i.e., any failure and maintenance properties will be ignored). This option can be used for "what-if" analyses to investigate the impact of a block on system metrics such as throughput, cost, etc. You can also set this option by selecting the block in the diagram and choosing Process Flow > Settings > Set Block as Failed.

      When this option is selected, no other properties will be available for the block; note, however, that any properties you have already specified are simply hidden because they are not relevant. The settings will reappear if you clear the Set block as failed option.

    • Duty cycle allows you to model situations where the actual usage of a block during system operation is not identical to the usage for which you have data (either from testing or from the field). This can include situations where the item:

      • Does not operate continuously (e.g., a DVD drive that was tested in continuous operation, but in actual use within a computer accumulates only 18 minutes of usage for every hour the computer operates).

      • Is subjected to loads that are greater than or less than the rated loads (e.g., a motor that is rated to operate at 1,000 rpm but is being used at 800 rpm).

      • Is affected by changes in environmental stress (e.g., a laptop computer that is typically used indoors at room temperature, but is being used outdoors in tropical conditions).

In these cases, continuous operation at the rated load is considered to be a duty cycle of 1. Any other level of usage is expressed as a percentage of the rated load value or operating time. For example, consider the DVD drive mentioned above; its duty cycle value would be 18 min / 60 min = 0.3. A duty cycle value higher than 1 indicates a load in excess of the rated value.

  • Consequential Costs allows you to choose or create models to represent costs that are always associated with the block. Cost per failure uses a cost model, and Downtime rate and Uptime rate use cost per unit time models. If no models are assigned, it is assumed that there are no additional costs.

  • Maintenance Group allows you to specify the maintenance group that the block belongs to.

  • State Change Triggers

    • Enable state change triggers (SCT) allows you to specify the starting state of the block (i.e., off or on) and its state upon repair, then specify events that will activate and/or deactivate the block during simulation. You can choose to activate or deactivate the block when items in specified maintenance groups go down or are restored. The current block does not need to be part of the specified maintenance group to use this functionality. This allows you to model a cold standby configuration (i.e., one where the component cannot fail when in standby) without using a standby container, which may be useful if you are using a parallel or complex configuration, as blocks can be connected only in series in standby containers.

      To add a state change trigger, click the Add icon in the Add a State Change Trigger field. The State Change Trigger window will appear, allowing you to define the trigger. Each trigger that you add will then be displayed in the Block Properties window. To edit an existing state change trigger, click the Edit icon for the state change trigger to open the State Change Trigger window.

  • Throughput
    • Units allows you to specify the units used for measuring input/output (mass) and time. For example, you might measure throughput in terms of kilograms per hour.
    • If the flow through the valve drops to or below the Switch threshold flow rate, then the valve switches from sending output to the primary (i.e., highest priority) flow path to the secondary path. To set priorities, select each connector that carries output from the valve and use the commands at Process Flow > Selection > Output Priority.

What's Changed? In previous versions, the switch threshold flow rate was called "minimum flow rate."

  • Maximum output defines, for each connected path, a maximum amount of output that can be sent to the path.

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