I have been meaning to write about this subject for some time. It may seem like a minor design consideration, but it can be one that brings a disproportionate amount of pain to project managers looking to close out a project.
Every year, I hear of a few instances where the factory-supplied solenoid valve on a day tank inlet is requested to be changed from an AC-powered model to a DC-powered model. The reason prompting the not-so-simple field exchange is usually attributed to a local inspector that has referenced “an NFPA requirement”. So, is a solenoid valve installed on the day tank inlet required to be DC powered? I say no, at least not due to “an NFPA requirement”. Here’s my interpretation of the often-confused NFPA requirement pertaining to solenoid valves on day tanks:
First, some basic facts: A day tank is used to provide a local (close proximity) fuel source to a diesel engine. In its most basic form, a day tank draws fuel from a bulk tank, and an engine draws fuel from the day tank. Industry-wide, a standard day tank is designed to receive input power from an AC circuit. The day tank control panel uses this AC power to control any tank-mounted supply and return pumps, as well as any inlet-mounted solenoid valve(s).
When day tanks are used, NFPA-110 (Ch. 18.104.22.168) requires that battery voltage be used to power any solenoid valve installed between the day tank and the engine. This battery voltage is typically obtained from the engine-starting batteries (12Vdc or 24Vdc).
It is important to note that the wording in NFPA-110 is referring to “Prime Mover Accessories”, and it should therefore NOT be considered applicable to any solenoid valve(s) installed upstream of the day tank.
Looking at the NFPA-110 requirement, the solenoid valve is described as the valve installed “from the day tank closest to the generator set” and the prime mover. The purpose of this solenoid valve is to open when the engine requires fuel, and to immediately shut off the fuel supply line when the “engine running signal” is removed. This solenoid “fuel shut off” valve is a safety feature commonly used back when engines had mechanical fuel pumps. In those days, the means to positively shut down the engine was to cut off the fuel supply and the intake air supply. Today’s engines use electronic fuel injection systems, making less relevant the use of a solenoid valve as a fuel cut-off valve. As applied today, a solenoid valve between the day tank and the engine serves mainly as an anti-siphon device, preventing fuel spills should a break occur in this fuel line.
If you agree that the intent for the NFPA-required solenoid valve is to stop the fuel supply to the engine, it is easy to see that shutting off a solenoid valve that is installed upstream of the day tank would not cut off the fuel supply to the engine, at least not until the day tank is depleted (a typical day tank holds sufficient fuel to run the engine for at least 2-hours).
You may not agree with my interpretation of the NFPA requirement, or you may simply wish to avoid a battle with local inspectors that demand DC-powered controls for any portion of the day tank and its accessories. If so, the best course of action would be to clearly specify such requirements in your project plans and specifications. A quality day tank vendor should have no problem with customizing the fuel day tank to meet a requirement for DC controls, or a DC-powered inlet solenoid valve, or to supply and coordinate a 24Vdc solenoid valve for installation between the day tank and engine. It is not difficult to design and build a day tank to have DC-powered controls and solenoid valves, but it is quite complicated to reconfigure a day tank in the field. In a nutshell, it is a much simpler task, when understood by the day tank manufacturer in advance.
I hope this article is helpful, if you ever find yourself with an inspectors’ objection to an AC-powered solenoid valve on a day tank inlet.