Master limited partnerships (MLP) are a great way of boosting your portfolio's income as they pay out the majority of their profits to unitholders or investors. However, the MLP space can be a minefield for inexperienced investors. While some MLPs may look like they offer high annualized distributions, in many cases these distributions can quickly change.

Indeed, the variable nature of MLP distributions means that a yield that looks attractive on an annualized basis can quickly deteriorate. This is because many MLPs operate within the commodity space, and as many investors have found out their detriment during the past few years, the commodity market can be highly volatile, making profits unpredictable.

Why are MLPs able to support such high yields?
With more reward comes more risk, and MLPs are no exception. MLPs are able to achieve such strong yields as their income is not subject to corporate income taxes. Instead, owners of an MLP are personally responsible for paying taxes on their individual portions of the MLP's income, gains, losses, and deductions, eliminating the double taxation generally applied to corporations. All in all, this means more cash is available for distributions. 

However, every unitholder is responsible for the taxes on his or her proportionate share of income, even if the MLP does not pay a cash distribution. In addition, although unitholders are usually limited in their liability, creditors of MLPs have the right to seek the return of distributions made to unitholders if the liability in question arose before the distribution was paid. This liability stays attached to the unitholder even after he or she sells the units.

So, overall despite the higher income, MLP units can be difficult instruments to own. 

Moreover, it is important to note that cash distributions are not guaranteed, and prospective investors need to evaluate whether or not the MLP can sustain its payout/distribution. 

So, after taking all of this into account I have arrived at a short list of three MLPs that look well placed to continue their payouts and reward investors far into the future. 

High yield, medium risk
I'll start with what is, in my opinion, the most risky of the three contenders in QR Energy L.P. (NYSE:QRE). Currently offering a quarterly distribution of $0.49 per unit, QR's annualized yield is around 11.9%. In addition, during the first half of this year, the company's cumulative distribution of $71 million was easily covered by $93 million in cash from operating activities.

During 2012, the partnership paid out nearly $100 million in distributions but generated $230 million in operating cash flow. So, overall, QR's distribution looks safe from a cash flow point of view. Furthermore, QR's income looks to be relatively predictable and sustainable for the next few years or even decades.

QR owns a selection oil and gas producing assets throughout the US. At the end of the second quarter, the partnership had 99 million barrels of proved reserves and was producing at a rate of 17, 264 barrels per day throughout the second quarter. This indicates that the partnership has enough oil for around 15 years of production at current rates, about the same as ExxonMobil. According to the partnership, the lifting cost is around $22 per barrel, so QR should be highly cash-generative for some time yet.

Low yield distribution businesses
The next two partnerships come as a pair due to their similar natural gas distribution operations. Cheniere Energy Partners L.P. (NYSEMKT:CQP) a subsidiary of Cheniere Energy and Crosstex Energy L.P. (NYSE:ENLK). Cheniere Energy Partners is the entity that actually owns and operates the Sabine Pass LNG terminal, among other assets. Currently, Cheniere offers unitholders a $0.43 quarterly payout for an annualized yield of 6.2%, and Crosstex offers partners a $0.33 quarterly distribution for an annualized yield of 6.5%.

Crosstex' unit distributions during the first half of this year totaled a cumulative $56 million, easily covered by the partnerships $62 million cash from operations. However, during the first half of this year Cheniere has not been able to cover its cumulative payout. Specifically, during the first half of this year, Cheniere made a cash flow loss from operations of $25 million and paid a cumulative $42 million in distributions to investors.

Having said that, Cheniere is currently in the process of constructing a liquefaction plant at its Sabine Pass terminal, which should, when it comes online with other assets during 2016, give the company yearly distributable cash flows of $1.29 billion, -- more than ten times the amount currently required to cover the current distribution. So, Cheniere's future payout looks safe, and there will be plenty of room for distribution growth.

Foolish summary
These three MLPs all off decent unit distributions that are well covered by cash flow and should add an income boost to your portfolio.  

Furthermore, as each partnership's distribution is well covered by cash flow, these payouts look safe and should not face sudden cuts. 

Fool contributor Rupert Hargreaves has no position in any stocks mentioned. The Motley Fool has no position in any of the stocks mentioned. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.