Setting the Record Straight on DeltaV SIS

One of the first, if not the first, weblogs in our world of process automation was started by Jim Pinto. He currently has eight weblogs covering several industrial automation suppliers.

On his Emerson Process Management weblog, a post from Thursday, June 12, 2008 made some assertions about DeltaV SIS that need clarification. I caught up with DeltaV SIS product manager Mike Boudreaux to set the record straight.

Assertion number one:

You can only at this moment transfer 8 digital bits of peer-to-peer between nodes. The Emerson sales person argued that this is all you need if you design your SIS nicely. Personally I seek to differ from this. I do want to go into an argument with him. This is a very expensive way via a pair of fiber optics cables to only have 8 bits. I heard that the real issue was due to a system design limitation.

Mike notes that that this misunderstanding comes from the fact that DeltaV SIS is unique for its distributed, modular architecture. A single logic solver module provides SIL 3-capable logic solving and 16 I/O channels that can be configured as digital input, digital output, HART analog input and HART two-state analog output. 32 logic solvers can comprise a single DeltaV SIS node.

On a single node, I/O data is published so that SIS Modules in any logic solver can reference the I/O input values from any other logic solver without any effect on the system response time. Additionally, each logic solver can publish up to eight secure parameters that any other logic solver can reference. This means that 512 I/O and 256 secure parameters can be published for use between logic solvers on a single node. A secure black channel communications protocol is used for this communication via a dedicated redundant peer-to-peer bus.

Beyond this local communications capability within a node, up to 32 DeltaV SIS nodes can be inter-connected via a dedicated redundant counter-rotating fiber optic ring (SISNet) that provides secure SIS communications between nodes. Up to 256 secure parameters can be published globally, and these global parameters can be referenced by SIS Modules in any logic solver that is on a node connected to the SISNet ring. This meets the secure communication requirements for most emergency shutdown, fire and gas and burner management safety applications. See more in the System Capacities section of DeltaV Books On-line.

Aside from the secure communications required to implement a safety instrumented function, there is no limit on the amount of non-secure data that can be passed to operator workstations for monitoring. This information is transmitted through the DeltaV Controller to the area control network, which is entirely separate from the secure DeltaV SIS SISNet communications.

Assertion number two:

The electronics takes only low powered inputs. You can only have at max 8xDOs (at max 500 mA per DO point) on a single DeltaV SIS module – which has 16 configurable IO points.

Each 24 VDC discrete output can provide up to 0.5 A of field power per channel, with a maximum of 4.0 A per logic solver card. DeltaV SIS is designed for implementing distributed safety logic. As such, inputs and outputs would typically be wired to the same logic solver for the SIF that is being controlled. With the general rule of 40% of SIS I/O being outputs, there are enough output channels provided to implement most safety instrumented functions (SIF).

In cases where more than 4.0 A is needed, external hardware such as the SIS Relay can be used to drive up to 16 DO’s from a single logic solver. Additionally, using the HART two-state AO channel can provide up to 16 output channels for operating a digital valve controller without any concern for field power limits. The Fisher DVC6000 SIS has recently been certified as suitable for use in SIL3 applications in 4-20 mA mode, with no need for a solenoid.

The final assertion:

It suits nicely into a small system type of expansion. At its gets larger and larger and becoming more complicated, it can become an engineering nightmare.

Scalability is the strength of the DeltaV SIS architecture. It’s well suited for large, geographically distributed applications throughout a facility. It has been applied on some of the largest floating production, storage and offloading system (FPSO) projects in the world. Fire & gas and emergency shutdown applications are required from bow to stern in these massive, floating oil and gas production facilities. Mike observes that the strength of using DeltaV SIS on large projects is the tight integration that is available with a similarly large DeltaV automation system.

I hope this helps set the record straight on this post, should you hear similar assertions being made as you evaluate safety instrumented systems in your IEC 61511 safety compliance efforts.

Update: Welcome readers of Gary Mintchell’s Feed Forward blog! I appreciate the visit and any comments you might have.

Posted Monday, June 23rd, 2008 under Safety.

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