What is a Node?

In order to study a system of a Process Hazard Analysis (PHA) or Hazard and Operability Study (HAZOPs), it must first be divided into nodes or sections for detailed review. Usually, these nodes include a certain section of piping, valves, and equipment. Deviations or what-if questions are then applied to each node to analyze the system.

How to Node a system?

The first step to noding is first taking time to understand the process. If possible read that through the process manual and study the PFD all the way through. If a PFD is available, always develop preliminary noding on the PFD, before moving on to highlight the P&IDs.

Noding should start at the beginning of the process and follow the succeeding flow path if possible. It is best practice to include any main process nodes first then followed by any utility, auxiliary, lube/seal oil/seal gas pump systems, chemical dosing, cooling towers, storage tanks/vessels, or any other non-main process sections.

Begin with the feed flow into the first vessel. Make the feed flow from another unit into its own separate node and do not include any vessels in this node. For the next node, highlight the first major vessel, then each of its outlet lines until they hit the next vessel. Include any minor equipment that is in between this vessel and the next. This includes pumps, heat exchangers, condensers, etc. Once you reach the next vessel, decide if you should start another new node or if it should be included in the previous node. This is discussed further in the Noding Tips section. Repeat this process for any vessels in the main process flow path. Minor process flows and side streams are noded and ordered in a way that makes sense. Sometimes it is best to include the nodes of a minor process flow in the middle of the main process flow nodes, rather than evaluating them at the end. For example, the main process flow of a heater would be the process material side. The combustion side is a minor flow path and would have its own node, but this node would be included right after the heater process flow side node, rather than after all the main process flow nodes.

Often, at the end of all major and minor process flow nodes, a Global node is included. A Global node is used to represent the whole process and serve as a place to put general causes or scenarios. This is where initiating events that affect more than one node can be addressed. This is also where external events such as global utility failure or weather events are best addressed. If separate human factors or facility siting checklists are not used, these scenarios are also included in the Global node.

The study facilitator or leader usually prepares the node list for a process prior to the start of the study. This helps to keep the PHA team focused during the study.

Too small or too big?

Nodes that are too big run the risk of missing causes and/or scenarios due to improper application of deviations. On the other hand, nodes that are too small will take unnecessary extra time for the PHA team since the team would have to go through the same issues/deviations repeatedly. As a rule of thumb try to limit the number of valves and pumps in any given node to a maximum of 5. For this rule, two pumps that are a main/spare combo are counted as one piece of equipment. For example, one node could have a main and spare pump and 4 control valves without going over the limit. This provides a good amount of causes per node, without there being too many.

Noding Tips

Below are examples and best practices for selecting nodes:

  1. Materials or phase can be a basis for separating nodes. Typically vapor and liquid phases should be separated into different nodes, unless these phase lines from the source vessel only consist of a pipe and a control valve that goes outside the unit, then it can be just combined with the vessel’s node. This is especially true for larger vessels, like towers, with multiple phases and outlet streams. Usually the bottoms, side draws, and overheaders are separated into different nodes.
  2. Feed flow from upstream (outside battery limit) and feed pump(s) into a Feed Surge Vessel should be a separate node. If there are heat exchangers along the way, include these as well (shell or tube).
  3. One major vessel/tank and pump(s) and possibly a small filter or train of heat exchangers. A major vessel is where the liquid level is controlled (e.g. fractionator tower, feed surge drum, tank, and its bottom along with its pump). Oftentimes larger vessels like towers, with multiple phases and outlet streams, are separated into different nodes. Usually, the vessel bottoms and the vessel itself are one node, while the side draw streams and overhead are separate nodes. An exception to this is if the overhead line of the vessel does not have any major equipment or systems, then it can be included with the major vessel.
  4. Distillation/fractionation overhead system & sour water is typically one node including the condensers, accumulator, product/reflux pump, and sour water pump. However, if the unit has its own sour water system, then the sour water boot of the accumulator, the pump, and lines should be included in the sour water node.
  5. Distillation/fractionation tower bottoms to storage are usually one node and the tower reboiler system is usually another node if the reboiler system includes a separate pump and fired heater.
  6. Distillation/fractionation tower side draws from a column are treated as a separate system and should be treated as one node.
  7. The heat exchanger tube side and shell side should be different nodes as they are normally both a part of the main flow but in different sections of the process. The exception to this is when the shell side or tube side is a cooling water line or steam line coming from a header that is not included in a separately noded system, then the shell and tube should be included in the same node.
  8. Fired heater, incinerator, or fired boiler combustion side which includes the firebox, ignition/fuel supply as well as combustion air supply is usually a separate node from the process node, but include right after the process node.
  9. A series of pumps for shipping/blending that has multiple suction lines and multiple discharge headers may be separated into their own node especially if they service multiple vessels or other nodes.
  10. A main pump’s spare is included in the main pump’s node. Pumps that are common spares (a spare for two different pumps) should be included in both of the main pump’s nodes.
  11. Compressor systems suction/discharge knockout drum, the compressor, and their level controller is usually one node.
  12. Chemical dosing/injection skids are typically vendor-related items and are treated as separate nodes. If the chemicals are simple and small (e.g. coagulants, corrosion inhibitor, etc.) with just small pumps and one or two vessels, these can be one node with multiple chemicals. These nodes go after all the main process nodes.
  13. Acid/caustic systems are typically separated in their own node because sometimes they feed to multiple units or systems and reverse flow scenarios should be verified. These nodes go after all the main process nodes.
  14. Lube/seal oil/seal gas systems of rotating equipment, typically large compressors and pumps, are treated as separate nodes and go after all the main process nodes. An exception to this is if the lube oil/seal oil/seal gas system does not contain major equipment related to the circulation of lube oil/ seal oil/seal gas then it will just be part of the rotating equipment node’s special operations deviation.
  15. Utility systems such as cooling water/feedwater systems, service/instrument air systems are treated as separate nodes and go after all the main process nodes.
  16. Filtration systems which include filters and pumps are usually grouped as one node. Filters operate liquid-filled (full of liquid) and therefore the inlet to the filter outlet, they should all be in the same node. If there are multiple trains of filter+pump combination, separate them so each filter+pump combination is a node.
  17. A cooling tower system is a utility system that includes all associated equipment such as the cooling tower, cooling water pumps, water make-up supply, etc. is treated as one node and goes after all the main process nodes.
  18. Storage tanks should be grouped together in the same node by tank type (either floating roof, fixed roof, or with vapor recovery) and material. The suction header from the tank to a multi-tank pump and discharge should be its own node. Headers that serve multiple tanks may be included in multiple nodes. Each tank’s suction lines and therefore the multi-tank header lines should be a part of the tank’s node.
  19. Storage LPG pressure vessels can be grouped and noded based on:

a.   PG Material stored (e.g., pentane, butane, propane, or a mixture of any)

b.   Vessel type (i.e., Sphere vs. bullets vs. hemisphere, etc.

PHA Prep Procedure