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Tales from the Silent Service – Nuclear Submarine Main Ballast Blow and Trim Systems

Most of the public have seen TV and movie scenes of a submarine being dived but aren’t privy to the actual mechanics and procedures of how it’s done. Typically, all we are shown is a camera-shot of the Captain ordering the dive of the boat then the familiar sound of the Klaxon “OOOH-GAH, OOOH-GAH,” a cut to the control room crew at their stations, then an external view of the sub submerging beneath the surface of the sea. But what happens, in reality, is this; the Captain, or in his absence his designated substitute, the Officer of the Deck (OD) orders the Chief of the watch (COOW) to announce the dive over the 1MC public address system; “Now rig ship for dive.” Then after all compartments have reported in the OD orders the Diving Officer (DO) to dive the submarine and he in turn orders the COOW to sound the diving alarm and announce the dive; “now dive, dive, dive” then he sounds the klaxon twice. Following the COOW opens the Main Ballast Tank (MBT) vents by actuating the two switches; one for forward and one for aft groups, located on the Ballast Control Panel (BCP) and that electro-mechanically activate and open the MBT vents. The vents are two large circular cutouts located in the tops of each MBT that allows the internal air to be released or “vented” from the MBTs thereby allowing tons of seawater to rush into them through flood large ports located at their bottoms. Next, the DO orders the Helmsmen/planesmen to put “10 degrees down angle on the stern planes, and make your depth 400 feet, steady as she goes” i.e.; maintain ordered course heading, and speed. The helmsmen and stern planesmen repeat back the orders and respond “aye” then execute the commands as the submarine gently slips beneath the waves. At that time the OD orders the COOW to close the vents and rig the control room for red lighting to prevent white light from beaming out through the periscope revealing the sub’s position at scope depth and preserving night vision for the control room crew viewing scopes, charts and gauges, and it will remain so for the duration of submerged operations. The submarine has now entered its realm and becomes a stealthy hunter-killer prowling the dark depths of the sea to places and adventures yet unknown.

The same scenario is somewhat repeated for surfacing the submarine with the primary difference being that the COOW activates the two electro-mechanically controlled “blow valves” with the main vents closed introducing high pressure air into the MBTs to force water out and giving the submarine positive buoyancy to assist it in being surfaced. However, prior to surfacing the OD “clears baffles” by performing a complete circling maneuver at about 200 feet to allow Sonar to listen and identify any close surface contacts in the immediate area. This is required because the sub is “sonar def” aft due to the noise of its own prop wash. Then, if clear of contacts the OD brings the boat to scope depth and he executes a 360-degree safety sweep of the horizon, even before the periscope fully breaks the surface, to visually confirm the absence of any contacts nearby that the sub could collide with. The surfacing maneuver is one of the most perilous times for a submarine along with transiting busy ports because of its low silhouette it can easily be overlooked and collide with a larger surface craft possibly being sent to the bottom.
Following is the basic operating theory of USN submarine Main Ballast Tank (MBT) Systems however the scenario and mechanics may vary between classes of submarines due to technological advancements and modernization. The MBT blow system is essentially three systems combined in that it’s comprised of the MBT Blow system, the Emergency (EMBT) blow system, and the Low-Pressure Blower system (LPB). These three systems can function either independently or jointly to bring the sub up to and remain on the sea surface under differing circumstances.
Their primary components are the Main Ballast Tanks which are a series of enormous saddle tanks constructed around the pressure hull of the ship and that are capable of ingesting or expelling tons of seawater to give the submarine either a positive, neutral or negative bouncy allowing it to be either submerged or surfaced. The tanks are linked together via the MBT Blow system comprised of high-pressure air flasks, piping, and valves that routes 3000 PSI high pressure air into the tanks to expel seawater to allow the vessel to surface. That may seem like a high volume of air, however, the seawater is expelled rather slowly at depth because of the immense sea pressure present i.e., sea pressure at 1000 feet is approximately 4400 PSI. So, when the sub is down deep it must be “driven” to the surface by the planesmen with the assistance of propulsion and a minimal positive bouncy provided by the blow system, so this can be a sluggish process initially. Then as the boat reaches shallower depths and sea pressure begins to decrease the rate of water being expelled from the tanks increases, as does positive buoyancy and the speed of ascent. However, under EMBT blow circumstances, although the physics remain the same the boat can reach speeds in excess of 30 knots, and travel the final 400 feet in seconds ultimately crashing through the sea surface in a hail of white water, its hull lurching as much as two-thirds of the way above the surface before settling back into the water. Its truly one hell of a ride for the crew and the only valid ticket is to wear submariner dolphins on one’s chest.
Under normal MBT blow conditions, sea pressure may exceed air pressure depending on depth, and piping and valves restrict the flow of air resulting in a minute reduction in efficiency of the air reaching the MBTs. And since every second is precious in an emergency situation under the sea, especially at deep depths the Emergency (EMBT) Blow System was devised after the tragic loss of USS Thresher SSN-593 with all 129 hands board in 1963. The system is central to a redesign program designated as The Submarine Safety Program “SUBSAFE” that covers all systems exposed to direct sea pressure and are critical to flooding recovery. In addition to the Emergency Blow System, SUBSAFE requires that all valves that are exposed to sea pressure be double valves as a backup in case one fails. Additionally, all work done and all materials used on those systems are strictly controlled to ensure the materials used in their assembly as well as the methods of assembly, maintenance, and testing are precise. They require certification with traceable quality evidence that tracks the item from the point of manufacture, including all records of the creation of the product to the point of installation within a SUBSAFE boundary.
In the case of Thresher, it was later determined that while conducting deep diving exercises off the Massachusetts coast she suffered a flooding emergency in her engine room caused by a failure of the brazing on a pipe joint. That resulted in high-pressure seawater spraying from on and shorting out one of the electrical panels in the engine room triggering an automatic emergency shutdown or “SCRAM” of the nuclear reactor which in turn prompted a loss of main propulsion. A Subsequent attempt to blow the ballast tanks failed and the cause was later attributed to excessive moisture in the submarine’s high-pressure air flasks that froze and blocked the airflow paths while passing through the blow valves’ strainers. The added weight of the flooded engine room caused the submarine to acquire a sharp up angle and stall, then she slipped stern first into the abyss passing crush depth, its pressure hull partially collapsing, and then crashing into the sea bed where the sub broke apart on impact. Sadly, she and her crew remain entombed there to this day fifty-eight years later.
Subsequently, the MBT blow system was improved to eliminate the icing problem and the emergency EMBT blow system was devised to assist a submarine to the surface more quickly in an emergency situation and act as a backup to the normal blow system by incorporating a methodology designated as “bank to tank.” That is when the two T-handle EMBT blow “chicken switches” located above the BCP in the control room are actuated they energize separate electro-hydraulic valves on individual HP air flasks located inside each MBT that open directly into the tanks bypassing all piping and other valving and that dumps 4500 PSI air directly from the air banks into each ballast tank giving the submarine a quick positive buoyancy in emergency situations to get the vessel to the surface as much as seven times more quickly than the standard blow system can.
The compressed air for the MBT blow and all other air systems on the submarine is provided by two large air compressors or HY-Paks that also furnish compressed air to the ships main hydraulic system accumulators and all other internal pneumatically operated systems through a series of reducers located throughout the submarine to bring the air pressure down to proper operating levels from 4500 PSI. For example, the hydraulic system uses 3000 PSI air, the torpedo tubes use 2000 lb. ram air, 250 lb. firing air and 600 lb. blow down air to empty the tubes of seawater post launching of a weapon or water slug.
The Low-Pressure Blower (LPB) works along with the MBT blow systems and has multiple other functions too. It is also hooked into the ventilation system to emergency evacuate compartments of smoke and poisonous gases and ingest fresh air into the boat at periscope depth. However, it is also linked to the MBT blow system and employed to blow the ballast tanks dry once the ship is surfaced. And, it can also be utilized from shallow depths to blow water from the MBTs to provide positive buoyancy when the submarine is being “driven” to the surface by the planesmen, and once on the surface from an LPB, normal MBT or EMBT blow left in operation for a time to blow the tanks “dry.” In this way compressed air is conserved and the air flasks remain fully charged in the event the ship must dive again rapidly. An EMBT blow can exhaust all the air flasks, but its an all or nothing situation in that case and so they can be recharged again if the submarine makes it to the surface. All these are all examples of the flexibility of a submarine’s blow systems that are dictated by mission requirements and other specific circumstances at hand.
The Trim system is a series of tanks connected by pumps and piping located throughout the hull that is contributory in keeping the ship in proper trim forward to aft and port to starboard. Maintaining athwart ship trim isn’t as challenging with a submarine as it is with surface vessels because a sub is designed and constructed so that it’s center of gravity located is below its center of buoyancy. This induces it to return to an even keel when it’s performing submerged high-speed maneuvers or is being rocked about on the surface. A 637 Class Fast Attack’s maximum roll was about 45 degrees at high speeds while submerged with full rudder on, and I have personally performed that maneuver as a helmsman. However, it will always swing back to near level port to starboard, and cannot be overturned under normal situations. As for fore and aft, a submarine rarely runs through the water in a dead level attitude. Rather they normally operate with either a slight up or down angle referred to as an up-bubble or down-bubble in reference to the carpenter’s type level bubble indicators located and monitored in the Control and Maneuvering Rooms. An up bubble is preferred under most conditions.
Another function of the trim system is to deter the boat from becoming either too heavy or light and keep it as close to neutral bouncy as possible in order to maintain desired depth more easily. Because when the boat is “light” it tends to rise requiring the planesmen to continually force it down to maintain ordered depth negatively affecting speed and producing needless noise from over operating the control devices. Contrastingly if it’s “heavy” it tends downward so the planesmen must do the opposite. Hence, in order to counter light condition seawater is pumped into the Main Trim Tank from sea to add weight or “ballast” to achieve more neutral buoyancy, and conversely to counteract light condition water is pumped back to sea. There are numerous forces that can affect a submarine’s buoyancy as it runs through the water. The first and possibly foremost is high speed because the force of the water against the keel, sail, and control surfaces tends to force the sub to rise, movement of personnel can affect both fore/aft and port/starboard trim. Cold and/or high salinity content water is denser and so tends to make the sub more buoyant and rise, while warmer water and/or lower salinity water and lower speeds can cause the boat to become heavier and sink when running through the sea.
Analogous to aircraft, submarines function on a three-dimensional plane that dictates the consideration of numerous physical and environmental aspects that present an array of operational challenges and can only be met by a highly trained and skilled crew in order to maintain proper and safe control of the vessel. It’s a team effort like no other.