Container ships move products from China to ports all over the globe. Sludges containing oil and water gradually build up on any relatively large ship as a byproduct of its lubrication and fuel oils used in electrical generation machinery and the main propulsion engine. This oil sludge presents operators with issues of safe disposal in compliance with maritime environmental regulations.
Oil sludge legally can be disposed of in two ways: 1) incineration in the vessel’s onboard incinerator; or 2) disposal to a barge or shore-based disposal facility, which can be expensive for the ship. A solution is to sell the oil sludge for conversion into a saleable product, allowed by some countries, such as China.
Vessels have two tanks for storage of oily waste: the oily bilge tank and the sludge tank. The sludge tank may be pumped to the oily bilge tank. The bilge holding tank may have trace oil residue contents, but is mechanically separated with a separate pump so the contents of the bilge holding tank are considered pure water for the purpose of calculating oil percentage.
Oil collected is based on the contents of the oily bilge tank and sludge tank. Oil sludge collectors pay for sludge, not water, and they require good quality ship sludge oil with less than 20 percent water.
Examples of payment for typical oily waste discharge with a payment coefficient of 35 percent of the value of heavy fuel oil (HFO) 380 at USD 650/metric ton (t) with different water percentages in the sludge:
- 80% water: 100 t x (1-.8) x USD 650/t x 0.35 = USD 4,550
- 20% water: 100 t x (1-.2) x USD 650/t x 0.35 = USD 18,200
As shown, the payment for sludge with less water is considerably more. However, current methods of determining the oil-to-water ratio of oil sludge and the related payment have to this point been guesswork and subject to manipulation.
A quality automatic fixed volume sampler for liquid and slurry offers a proven way to take a repeatable sample for analysis of oil-to-water ratio and is a superior method for accurately sampling oil sludge and water content to arrive at a fair and verifiable payment. It operates using the following steps:
- A joint sounding, in which a measurement tube is extended into the ship’s bunker oil tank to indicate the fullness level of the tank, is conducted with the crew to estimate the total quantity of sludge to be discharged and to determine the sampling rate.
- A spool piece with an automatic sampler that takes a representative 8 cc sample is mounted on the sludge discharge pipe.
- Periodic samples are taken over the course of the sludge discharge to arrive at a composite representative sample of the entire sludge discharge. A serialized tamper seal is applied to the sample container in the presence of the crew.
- The sample is sent to a laboratory approved by the owner, where the representative sample is homogenized and then tested to determine the water content.
- Total discharge quantity x (1-water percentage) x price of HFO 380 x payment coefficient = Total customer payment
Example: With an estimated pumping rate of 15 cubic meters (m3) per hour, and when the following tank quantities are observed via joint sounding with the crew:
- Sludge tank: 70 m3
- Oily bilge tank: 20 m3
- Dirty oil tank: 10 m3
Total quantity = 100 m3, and estimated discharge time is seven hours.
Based on the desire to obtain a sample of approximately 8 cc, the sampling rate is determined: one sample every four minutes results in approximately 800 cc of sample at the conclusion of pumping. If the duration of pumping is faster, the sampling rate is increased.
By recording the start and stop pumping times on the various tanks, the individual pumping rates of each tank can be calculated, and the sample may be corrected mathematically to ensure it remains representative, even if the flow rates on the tanks differ significantly. Since the same pump is generally used to pump all oily waste tanks with similar flow rates, mathematically correcting the sample is seldom required.
The sampling solution for this challenge is the Sentry® ISOLOK® SAA fixed volume sampler for liquid and slurry. The Sentry SAA offers ease of use and convenience, as all components may be hand carried by the surveyor in a backpack to the vessel and set up within 15 minutes. The components include:
1. Four-inch spool piece with the sample collection orifice
2. ATEX-certified Sentry ISOLOK SAA automatic fixed volume sampler
3. Sampler controller
4. Pneumatic hose for vessel LP air
5. Extension cord for 220 V/60 Hz
6. Sample collection container
7. Serialized tamper seals
The Sentry ISOLOK SAA sampler features a pneumatically actuated plunger with a solenoid valve. When the plunger is inserted into the collection orifice, it collects a precise 8 cc of sample that gravity drains into the sample collection container. The controller actuates the solenoid to allow for programming of the sampling interval. The spool piece is built with an angled pipe to ensure full bore flow is maintained continuously. This same system is currently employed in many oil and gas applications with similar sediment-containing fluids over the same viscosity range of water and oil sludge.
Accurate payment for oil sludge starts with a reliable, repeatable sample, which only the Sentry ISOLOK SAA automatic fixed volume sampler can offer.
Learn more about the Sentry ISOLOK SAA sampler and related sampling solutions here.