Process and Utility Controls in the Sea Freight Industry
As the poetic analogy goes, if humans live one breath at a time, international trade lives one cargo ship at a time. Each year, an estimated 11 billion tons of goods make their way over open ocean, dutifully transported by over 100,000 active seafaring freight vessels between ports across the world. These ships and their cargo keep local economies churning, delivering resources and goods to eagerly awaiting markets that would be materially impacted should a shipment be delayed.
This presents the imperative of reliability on cargo vessels, through all of the individual systems within these ships that together make sure they reach their final destinations safely, without fail. Hundreds of discrete mechanical, fluid, compressed gas, and electronic systems can be found in any given cargo vessel, each with its own operational nuances that necessitate having robust controls and instrumentation.
The design and function of shipboard systems have many similarities to industrial process and utility systems, but with key differences namely in their resilience, redundancy, environmental compatibility, and approvals. Maritime systems have to stand up to harsh conditions and must perform under the pressure of emergency situations.
Even when conditions are relatively calm, systems that experience drift or repeatability error can stack up inaccuracies that will lead to surprise issues – one good example being poor fuel consumption measurement that can leave a vessel with low fuel levels before it reaches port.
Life and Safety systems are especially sensitive to dependability issues, where system failure is simply not an option. In all cases, knowing what to look for in instrumentation selection for marine applications – and then properly maintaining that instrumentation over time – makes all the difference in assuring that ship systems perform as expected.
Maritime Freight Instrumentation Application Examples
Seafaring ships use virtually all instrumentation types found in industrial applications, including flow, pressure, temperature, vacuum, vibration, level, and position sensors. Examples include:
- Equipment Hydraulic Systems for cargo lifts, positioning cranes, and access gangways
- Central Vessel Hydraulic Systems such as powered door, partition, and hatch operators
- Potable Cold and Hot Water Systems for crew facilities
- Ballast Control
- Emergency Deluge
- Steering and Positioning Systems such as rudder control, anchor drives, and windlasses
- Heavy Fuel Storage and Distribution
- Pneumatic Conveyance
- Engine Lubrication
- Life and Safety Systems such as life raft launch, fire suppression, and emergency oxygen delivery
- Onboard Cargo Management Systems such as fuel bunkering, material transfer, and cargo weighing
In addition, advanced analytic sensors can also be found in quality-controlled applications, including freshwater treatment, septic treatment, and emissions control systems.
Selecting Sensors for Marine Applications
Instrumentation intended to serve at sea should be carefully selected with the intended application and environmental conditions in mind. Key selection criteria include:
Stainless Steel Type 316 is the standard choice for housing materials that will be exposed to seawater and high-salinity vapors. For ultra-corrosive, long-term, or continuously wetted saltwater contact, exotic alloys such as titanium, Hastelloy, and CuNiFe (a copper, nickel, iron blend) are good alternatives.
Wiring connections and enclosures should be IP68 or NEMA 6P rated for marine applications. These ratings offer total dust ingress, complete and permanent submersion, and anti-corrosion protection. IP68 rated instruments typically have molded cordsets, and we recommend that this wiring is run back to the primary termination point to reduce field connections. That said, also consider the need for fast / emergency replacement, which may suggest termination to a field controller or input rack, enclosed within an IP68 rated enclosure, near enough to the point of use that urgent replacement is not prohibitive.
Fluid marine systems tend to be high pressure, calling for appropriately-rated BSP, NPT, or G threaded piping connections. Assure that fitting specifications are properly coordinated, so as not to mix imperial and metric threads. For lower pressure applications, quick-disconnect or swaged fittings are good selections for easy replacement. Large diameter connections are often 150#, 300#, or 600# bolted flanges, which also provide structural capacity for carrying the weight of large instrumentation (such as fuel mass flow meters).
Wetted instrument materials must be selected for compatibility with the media types that they will contact. For example with seawater, ceramic sensor diaphragms, and FKM or NBR sealing elastomers, should be used. For fuel, compressed gas, hydraulic, and other media, materials must be selected for the exact chemistry, operating pressure, and temperature expected.
Testing and Approvals
Two topics to discuss here: application approvals, and marine classifications. For application approvals, various special use-cases require testing and approvals for that use, such as intrinsically safe and class/division electrical ratings for explosive environments. For marine classifications, there are over 70 organizations worldwide that provide technical standards that govern the construction and operation of marine ships and their systems, which are selectively required by flag states for ships operating in their waters. Depending on the application and port, instruments may need individual marine approvals, or approvals may be provided at the system or full-ship level.
Reliability and Repeatability
It goes without saying that any component put into service in a marine application needs to perform as expected, as even seemingly simple repairs or replacement can prove to be quite harrowing at sea. Select sensors with published performance, repeatability, accuracy, and response values, and with proven efficacy in extreme conditions. Also, look for sensors with high peak or surge condition capacity (as high as two times nominal values).
No matter how good a single component may be, analyze all applications for probabilistic failure scenarios that can be addressed by redundant control components. Not just for sensors, but for all critical pieces in a control loop. 1oo2 (“one out of two”) failure tolerance should be the standard baseline design for critical systems, which means that one out of two redundant components in a loop can fail while the circuit continues to operate safely.
Considering that ocean cargo vessels tend to operate far away from local supply houses, carrying spare components onboard at all times is a matter of course. Further, preventative maintenance such as calibrations, accuracy checks against certified test sensors, and intentionally swapping sensors on regular intervals are all good measures to assure that sensor performance is not left to chance.
As a veteran-owned small business, Whitman Controls is dedicated to supplying premium quality, reliable, technologically advanced instrumentation for use in nearly any application. Our Bristol, CT manufacturing facility embodies over 40 years of engineering, fabrication, and customer service expertise, serving both end-user and manufacturing customers nationwide through direct and distribution channels.
Our values drive us to provide the highest level of servant partnership that you can find. To discuss your applications or to learn more about our capabilities, please contact us at (800) 233-4401, via email at [email protected], or online at www.whitmancontrols.com.